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FINAL
TURBIDITY TOTAL MAXIMUM DAILY LOADS FOR SULPHUR
CREEK, OKLAHOMA (OK410600010030_00)
Prepared for:
OKLAHOMA DEPARTMENT OF ENVIRONMENTAL QUALITY
Prepared by:
AUGUST 2010
FINAL
TURBIDITY TOTAL MAXIMUM DAILY LOADS FOR SULPHUR
CREEK, OKLAHOMA (OK410600010030_00)
OKWBID
OK410600010030_00
Prepared for:
OKLAHOMA DEPARTMENT OF ENVIRONMENTAL QUALITY
Prepared by:
8000 Centre Park Drive, Suite 200
Austin, TX 78754
AUGUST 2010
Oklahoma Department of Environmental Quality: FY07/08 106 Carryover Grant (I-006400-08)
Funding for the development of this TMDL Report was provided through a federal Clean Water Act grant to
the Oklahoma Department of Environmental Quality from the U.S. Environmental Protection Agency.
Sulphur Creek TMDL Table of Contents
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TABLE OF CONTENTS
EXECUTIVE SUMMARY ................................................................................................. ES-1
SECTION 1 INTRODUCTION ............................................................................................. 1-1
1.1 TMDL Program Background ..................................................................................... 1-1
1.2 Watershed Description ............................................................................................... 1-4
1.3 Stream Flow Data ....................................................................................................... 1-7
SECTION 2 PROBLEM IDENTIFICATION AND WATER QUALITY TARGET ...... 2-1
2.1 Oklahoma Water Quality Standards ........................................................................... 2-1
2.2 Problem Identification ................................................................................................ 2-3
2.3 Water Quality Target .................................................................................................. 2-4
SECTION 3 POLLUTANT SOURCE ASSESSMENT ....................................................... 3-1
3.1 NPDES-Permitted Facilities ....................................................................................... 3-1
3.1.1 Continuous Point Source Discharges ............................................................. 3-2
3.1.2 Concentrated Animal Feeding Operations ..................................................... 3-2
3.1.3 Stormwater Permits for MA4 and Construction Activities ............................ 3-2
3.1.4 Section 404 permits ........................................................................................ 3-2
3.2 Nonpoint Sources ....................................................................................................... 3-3
SECTION 4 TECHNICAL APPROACH AND METHODS .............................................. 4-1
4.1 Determining a Surrogate Target ................................................................................. 4-1
4.2 Using Load Duration Curves to Develop TMDLs ..................................................... 4-4
4.3 Development of Flow Duration Curves ..................................................................... 4-4
4.4 Development of TMDLs Using Load Duration Curves ............................................. 4-6
SECTION 5 TMDL CALCULATIONS ................................................................................ 5-1
5.1 Estimated Loading and Critical Conditions ............................................................... 5-1
5.2 Wasteload Allocation ................................................................................................. 5-2
5.2.1 Section 404 permits ........................................................................................ 5-2
5.3 Load Allocation .......................................................................................................... 5-3
5.4 Seasonal Variability .................................................................................................... 5-3
5.5 Margin of Safety ......................................................................................................... 5-3
5.6 TMDL Calculations .................................................................................................... 5-3
5.7 Reasonable Assurances .............................................................................................. 5-4
SECTION 6 PUBLIC PARTICIPATION ............................................................................ 6-1
SECTION 7 REFERENCES .................................................................................................. 7-1
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APPENDICES
Appendix A Ambient Water Quality Data 1991 - 2007
Appendix B Projected Flow exceedance frequencies for Sulphur Creek Flow Duration Curve
Appendix C State of Oklahoma Antidegradation Policy
Appendix D Response to Public Comments
LIST OF FIGURES
Figure 1-1 Watersheds Not Supporting Fish and Wildlife Propagation Use
within the Study Area ........................................................................................... 1-3
Figure 1-2 Land Use Map by Watershed ............................................................................... 1-6
Figure 3-1 Locations of Permitted Facilities in the Study Area ............................................. 3-4
Figure 4-1 Linear Regression for TSS-Turbidity under Base-flow Conditions for Sulphur
Creek (OK 410600010030_00) ............................................................................ 4-3
Figure 4-2 Flow Duration Curve for Sulphur Creek (OK410600010030_00) ....................... 4-6
Figure 5-1 Load Duration Curve for Total Suspended Solids in Sulphur Creek
(OK410600010030_00) ........................................................................................ 5-2
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LIST OF TABLES
Table ES-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List ................................................................................. ES-2
Table ES-2 Summary of Turbidity Samples Collected During Base Flow Conditions 1998 -
2007 ................................................................................................................... ES-2
Table ES-3 Summary of TSS Samples Collected During Base Flow Conditions 1998 -
2007 ................................................................................................................... ES-3
Table ES-4 Turbidity TMDLs based on Total Suspended Solids Calculations for Sulphur
Creek (OK410600010030_00) .......................................................................... ES-6
Table 1-1 Water Quality Monitoring Stations used for 2008 303(d) Listing Decision ........ 1-2
Table 1-2 County Population and Density ............................................................................ 1-4
Table 1-3 Average Annual Precipitation by Watershed ....................................................... 1-4
Table 1-4 Land Use Summaries by Watershed ..................................................................... 1-5
Table 2-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List .................................................................................... 2-2
Table 2-2 Summary of All Turbidity Samples 1991 - 2007 ................................................. 2-3
Table 2-3 Summary of Turbidity Samples Collected During Base Flow Conditions 1992 -
2007 ...................................................................................................................... 2-3
Table 2-4 Summary of All TSS Samples 1991 - 2007 ......................................................... 2-4
Table 2-5 Summary of TSS Samples Collected During Base Flow Conditions 1992 -
2007 ...................................................................................................................... 2-4
Table 3-1 Stormwater Permits for Construction Activities .................................................. 3-2
Table 5-1 Turbidity TMDL based on Total Suspended Solids Calculations for Sulphur Creek
(OK410600010030_00) ........................................................................................ 5-4
Table 5-2 Partial List of Oklahoma Water Quality Management Agencies ......................... 5-5
Sulphur Creek TMDL Acronyms and Abbreviations
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ACRONYMS AND ABBREVIATIONS
BMP best management practice
CAFO Concentrated Animal Feeding Operation
CFR Code of Federal Regulations
cfs Cubic feet per second
CPP Continuing planning process
CWA Clean Water Act
DMR Discharge monitoring report
IQR interquartile range
LA Load allocation
LDC Load duration curve
LOC line of organic correlation
mg Million gallons
mgd Million gallons per day
mg/L microgram per liter
MOS Margin of safety
MS4 Municipal separate storm sewer system
NPDES National Pollutant Discharge Elimination System
NRCS National Resources Conservation Service
NTU nephelometric turbidity unit
OLS ordinary least square regression
O.S. Oklahoma statutes
ODAFF Oklahoma Department of Agriculture, Food and Forestry
ODEQ Oklahoma Department of Environmental Quality
OPDES Oklahoma Pollutant Discharge Elimination System
OWRB Oklahoma Water Resources Board
PRG Percent reduction goal
TMDL Total maximum daily load
TSS Total suspended solids
USDA U.S. Department of Agriculture
USEPA U.S. Environmental Protection Agency
USGS U.S. Geological Survey
WLA Wasteload allocation
WQM Water quality monitoring
WQS Water quality standard
WWTP Wastewater treatment plant
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EXECUTIVE SUMMARY
This report documents the data and assessment used to establish a TMDL for Sulphur
Creek, a tributary of the Blue River. The 2008 Integrated Water Quality Assessment Report
(Oklahoma Department of Environmental Quality [ODEQ] 2008) identified Sulphur Creek as
impaired for turbidity. Data assessment and TMDL calculations are conducted in accordance
with requirements of Section 303(d) of the CWA, Water Quality Planning and Management
Regulations (40 CFR Part 130), USEPA guidance, and ODEQ guidance and procedures.
ODEQ is required to submit all TMDLs to USEPA for review and approval. Once the USEPA
approves a TMDL, the waterbody may be moved to Category 4a of a state’s Integrated Water
Quality Monitoring and Assessment Report, where it remains until compliance with water
quality standards (WQS) is achieved (USEPA 2003).
The purpose of this TMDL report is to establish pollutant load allocations for turbidity in
impaired waterbodies, which is the first step toward restoring water quality. TMDLs determine
the pollutant loading a waterbody can assimilate without exceeding the WQS for that pollutant.
TMDLs also establish the pollutant load allocation necessary to meet the WQS established for a
waterbody based on the relationship between pollutant sources and in-stream water quality
conditions. A TMDL consists of a wasteload allocation (WLA), load allocation (LA), and a
margin of safety (MOS). The WLA is the fraction of the total pollutant load apportioned to
point sources, and includes stormwater discharges regulated under the National Pollutant
Discharge Elimination System (NPDES) as point sources. The LA is the fraction of the total
pollutant load apportioned to nonpoint sources. The MOS is a percentage of the TMDL set
aside to account for the lack of knowledge associated with natural process in aquatic systems,
model assumptions, and data limitations.
This report does not stipulate specific control actions (regulatory controls) or management
measures (voluntary best management practices) necessary to reduce turbidity loadings within
each watershed. Watershed-specific control actions and management measures will be
identified, selected, and implemented under a separate process involving stakeholders who live
and work in the watershed; tribes; and local, state, and federal government agencies.
E.1 Problem Identification and Water Quality Target
The TMDL in this report address fish and wildlife propagation for the subcategory warm water
aquatic community. Table ES-1, an excerpt from Appendix B of the 2008 Integrated Report
(ODEQ 2008), summarizes the warm water aquatic community use attainment status and the
scheduled date for TMDL development established by ODEQ for the impaired waterbody of
the Study Area.
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Table ES-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List
Waterbody ID Waterbody Name
Stream Miles
Category
TMDL Date
Priority
Warm Water
Aquatic
Community
OK410600010030_00 Sulphur Creek 14.6 5a 2013 2 N
N = Not Supporting;
5a = TMDL is underway or will be scheduled
Source: 2008 Integrated Report, ODEQ 2008
The data in Table ES-2 were used to support the decision to place Sulphur Creek on the
ODEQ 2008 303(d) list (ODEQ 2008 for nonsupport of the Fish and Wildlife Propagation use
based on turbidity levels observed in the waterbody. Turbidity is a measure of water clarity
and is caused by suspended particles in the water column. Because turbidity cannot be
expressed as a mass load, total suspended solids (TSS) are used as a surrogate in this TMDL.
Therefore, both turbidity and TSS data are presented to support TMDL development.
The numeric criteria for turbidity to maintain and protect the use of “Fish and Wildlife
Propagation” from Title 785:45-5-12 (f) (7) is as follows:
(A) Turbidity from other than natural sources shall be restricted to not exceed the following
numerical limits:
1. Cool Water Aquatic Community/Trout Fisheries: 10 NTUs;
2. Lakes: 25 NTU; and
3. Other surface waters: 50 NTUs.
(B) In waters where background turbidity exceeds these values, turbidity from point sources
will be restricted to not exceed ambient levels.
(C) Numerical criteria listed in (A) of this paragraph apply only to seasonal base flow
conditions.
(D) Elevated turbidity levels may be expected during, and for several days after, a runoff event.
Table ES-2 Summary of Turbidity Samples Collected During Base Flow Conditions
1998 - 2007
WQM Station
Number of Turbidity
Samples Collected
During Base Flow
Conditions
Number of
Samples
Exceeding 50 NTU
during Base Flow
Conditions
Percentage of
Samples
Exceeding
Criterion
during Base
Flow
Conditions
Average Turbidity
(NTU) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 19 2 11% 34
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Table ES-3 presents a subset of total suspended solids data for samples collected during
base flow conditions. Water quality data for turbidity and TSS are provided in Appendix A.
Table ES-3 Summary of TSS Samples Collected During Base Flow Conditions
1998 -2007
WQM Station
Number of TSS
Samples Collected
During Base Flow
Conditions
Average TSS
(mg/L) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 18 19
The Code of Federal Regulations (40 CFR §130.7(c)(1)) states that, “TMDLs shall be
established at levels necessary to attain and maintain the applicable narrative and numerical
water quality standards.” An individual water quality target established for turbidity must
demonstrate compliance with the numeric criteria prescribed in the Oklahoma WQS
(OWRB 2008). According to the Oklahoma WQS [785:45-5-12(f)(7)], the turbidity criterion
for streams with warm water aquatic community (WWAC) beneficial use is 50 NTUs
(OWRB 2008). The turbidity of 50 NTUs applies only to seasonal base flow conditions.
Turbidity levels are expected to be elevated during, and for several days after, a storm event.
TMDLs for turbidity in streams designated as warm water aquatic community must take
into account that no more than 10 percent of the samples may exceed the numeric criterion of
50 NTU. However, as described above, because turbidity cannot be expressed as a mass load,
TSS is used as a surrogate in this TMDL. Since there is no numeric criterion in the Oklahoma
WQS for TSS, a specific method must be developed to convert the turbidity criterion to TSS
based on a relationship between turbidity and TSS. The method for deriving the relationship
between turbidity and TSS, and for calculating a water body specific water quality target using
TSS, is summarized in Section 4 of this report.
E.2 Pollutant Source Assessment
A pollutant source assessment characterizes known and suspected sources of pollutant
loading to impaired waterbodies. Sources within a watershed are categorized and quantified to
the extent that information is available. Turbidity may originate from NPDES-permitted
facilities, fields, construction sites, quarries, stormwater runoff and eroding stream banks. The
2008 Integrated Water Quality Assessment Report (ODEQ 2008) listed potential sources of
turbidity in Sulphur Creek (OK410600010030_00) as grazing in riparian corridors of streams
and creeks, highway/road/bridge runoff (non-construction related), non-irrigated crop
production, rangeland grazing, and other unknown sources.
There are no NPDES-permitted facilities and no municipal separate storm sewer systems
or CAFOs in the Study Area.
The relative homogeneous land use/land cover categories within the Study Area are
associated with agricultural and range management activities. This suggests that various
nonpoint sources of TSS include sediments originating from grazing in riparian corridors of
streams and creeks, highway/road/bridge runoff (non-construction related), non-irrigated crop
production, rangeland grazing and other sources of sediment loading (ODEQ 2008). Elevated
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turbidity measurements can be caused by stream bank erosion processes, stormwater runoff
events and other channel disturbances. However, there is insufficient data available to quantify
contributions of TSS from these processes. TSS or sediment loading can also occur under non-runoff
conditions as a result of anthropogenic activities in riparian corridors which cause
erosive conditions. Sediment loading of streams can also originate from natural erosion
processes, including the weathering of soil, rocks, and uncultivated land; geological abrasion;
and other natural phenomena. Given the lack of data to establish the background conditions for
TSS/turbidity, separating background loading from nonpoint sources is not feasible in this
TMDL development.
E.3 Using Load Duration Curves to Develop TMDLs
Turbidity is a commonly measured indicator of the suspended solids load in streams.
However, turbidity is an optical property of water, and measures scattering of light by
suspended solids and colloidal matter. To develop TMDLs, a gravimetric (mass-based)
measure of solids loading is required to express loads. There is often a strong relationship
between the total suspended solids concentration and turbidity. Therefore, the TSS load, which
is expressed as mass per time, is used as a surrogate for turbidity and represents the maximum
one-day load the stream can assimilate while still attaining the WQS.
To determine the relationship between turbidity and TSS, a linear regression between TSS
and turbidity was developed using data collected from 1998 to 2007 at one station within the
Study Area. Prior to developing the regression the following steps were taken to refine the
dataset:
Assign values to censored data (i.e., measured concentrations lower than the analytical
quantitation limit and, thus, reported as less than the quantitation limit). For example,
using 9.99 to replace all samples reported as “<10”;
Remove data collected under high flow conditions exceeding the base-flow criterion.
This means that measurements corresponding to flow exceedance frequencies lower
than 25% were not used in the regression;
Check rainfall data on the day when samples were collected and on the previous two
days. If there was a significant rainfall event (>= 1.0 inch) in any of these days, the
sample will be excluded from regression analysis with one exception. If the significant
rainfall happened on the sampling day and the turbidity reading was less than 25 NTUs
(half of turbidity standard for streams), the sample will not be excluded from analysis
because most likely the rainfall occurred after the sample was taken; andLog-transform
both turbidity and TSS data to minimize effects of their non-linear data
distributions.
The TMDL calculations presented in this report are derived from load duration curves
(LDC). LDCs facilitate rapid development of TMDLs, and as a TMDL development tool, are
effective at identifying whether impairments are associated with point or nonpoint sources.
The basic steps to generating an LDC involve:
obtaining daily flow data for the site of interest from the USGS (or project flow using
Oklahoma TMDL Toolbox if station is ungaged);
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sorting the flow data and calculating flow exceedance frequencies for the time period
and season of interest;
obtaining available turbidity and TSS water quality data;
matching the water quality observations with the flow data from the same date;
displaying a curve on a plot that represents the allowable load multiplying the actual or
estimated flow by the WQtarget for TSS;
multiplying the flow by the water quality parameter concentration to calculate daily
loads (for sampling events with both TSS and turbidity data, the measured TSS value
is used; if only turbidity was measured, the value was converted to TSS using the
regression equation in Figure 4-1); then
plotting the flow exceedance frequencies and daily load observations in a load duration
plot.
The culmination of these steps is expressed in the following formula, which is displayed on
the LDC as the TMDL curve:
TMDL (lb/day) = WQtarget * flow (cfs) * unit conversion factor
where: WQtarget = 31 mg/L (TSS)
unit conversion factor = 5.39377 L*s*lb /(ft3*day*mg)
The flow exceedance frequency (x-value of each point) is obtained by looking up the
historical exceedance frequency of the measured or estimated flow; in other words, the percent
of historical observations that equal or exceed the measured or estimated flow. Historical
observations of TSS and/or turbidity concentrations are paired with flow data and are plotted on
the LDC. The TSS load (or the y-value of each point) is calculated by multiplying the TSS
concentration (measured or converted from turbidity) (mg/L) by the instantaneous flow (cfs) at
the same site and time, with appropriate volumetric and time unit conversions. TSS loads
representing exceedance of water quality criteria fall above the water quality criterion line.
E.4 TMDL Calculations
The objective of a TMDL is to estimate allowable pollutant loads and to allocate these
loads to the known pollutant sources in the watershed so appropriate control measures can be
implemented and the WQS achieved. A TMDL is expressed as the sum of three elements as
described in the following mathematical equation:
TMDL = Σ WLA + Σ LA + MOS
The WLA is the portion of the TMDL allocated to existing and future point sources. The
LA is the portion of the TMDL allocated to nonpoint sources, including natural background
sources. The MOS is intended to ensure that WQS will be met. Thus, the allowable pollutant
load that can be allocated to point and nonpoint sources can then be defined as the TMDL
minus the MOS.
The overall Percent Reduction Goal (PRG) is calculated as the reduction in load required
so no more than 10 percent of the samples collected under base-flow conditions would exceed
TMDL targets for TSS. The PRG for Sulphur Creek is calculated to be 11.7 percent.
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The maximum assimilative capacity of a stream depends on the flow conditions of the
stream. The higher the flow is, the more wasteload the stream can handle without violating
water quality standards. Thus, the TMDL, WLA, LA, and MOS will vary with flow condition,
and are calculated at every 5th flow interval percentile (Table ES-4).
Table ES-4 Turbidity TMDLs based on Total Suspended Solids Calculations for
Sulphur Creek (OK410600010030_00)
Percentile
Flow
(cfs)
TMDL
(lb/day)
WLA (lb/day) LA
(lb/day)
MOS
WWTP MS4 Growth (lb/day)
0 1,668 NA 0 0 NA NA NA
5 45 NA 0 0 NA NA NA
10 19 NA 0 0 NA NA NA
15 12 NA 0 0 NA NA NA
20 9.2 NA 0 0 NA NA NA
25 7.4 1,253 0 0 12.5 1,115 125
30 6 1,016 0 0 10.2 904 102
35 5 847 0 0 8.5 754 85
40 4.2 711 0 0 7.1 633 71
45 3.7 627 0 0 6.3 558 63
50 3.2 542 0 0 5.4 482 54
55 2.7 457 0 0 4.6 407 46
60 2.3 390 0 0 3.9 347 39
65 2.1 356 0 0 3.6 317 36
70 1.8 305 0 0 3.0 271 30
75 1.6 271 0 0 2.7 241 27
80 1.4 237 0 0 2.4 211 24
85 1.2 203 0 0 2.0 181 20
90 1 169 0 0 1.7 151 17
95 0.7 119 0 0 1.2 106 12
100 0 0 0 0 0 0 0
E.5 Reasonable Assurance
ODEQ will collaborate with a host of other state agencies and local governments working
within the boundaries of state and local regulations to target available funding and technical
assistance to support implementation of pollution controls and management measures. Various
water quality management programs and funding sources provide a reasonable assurance that
the pollutant reductions as required by this TMDL can be achieved and water quality can be
restored to maintain designated uses. ODEQ’s Continuing Planning Process (CPP), required by
the CWA §303(e)(3) and 40 CFR 130.5, summarizes Oklahoma’s commitments and programs
aimed at restoring and protecting water quality throughout the state (ODEQ 2006). The CPP
can be viewed from ODEQ’s website at 2006 Continuing Planning Process. Table 5-2 provides
a partial list of the state partner agencies ODEQ will collaborate with to address point and
nonpoint source reduction goals established by TMDLs.
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SECTION 1
INTRODUCTION
1.1 TMDL Program Background
Section 303(d) of the Clean Water Act (CWA) and U.S. Environmental Protection Agency
(USEPA) Water Quality Planning and Management Regulations (40 Code of Federal
Regulations [CFR] Part 130) require states to develop total maximum daily loads (TMDL) for
waterbodies not meeting designated uses where technology-based controls are in place.
TMDLs establish the allowable loadings of pollutants or other quantifiable parameters for a
waterbody based on the relationship between pollution sources and in-stream water quality
conditions, so states can implement water quality-based controls to reduce pollution from point
and nonpoint sources and restore and maintain water quality (USEPA 1991).
This report documents the data and assessment used to establish a turbidity TMDL for
Sulphur Creek, a tributary of the Blue River. The 2008 Integrated Water Quality Assessment
Report (Oklahoma Department of Environmental Quality [ODEQ] 2008) identified Sulphur
Creek as impaired for turbidity. Data assessment and TMDL calculations are conducted in
accordance with requirements of Section 303(d) of the CWA, Water Quality Planning and
Management Regulations (40 CFR Part 130), USEPA guidance, and ODEQ guidance and
procedures. ODEQ is required to submit all TMDLs to USEPA for review and approval. Once
the USEPA approves a TMDL, the waterbody may be moved to Category 4a of a state’s
Integrated Water Quality Monitoring and Assessment Report, where it remains until
compliance with water quality standards (WQS) is achieved (USEPA 2003).
The purpose of this TMDL report is to establish pollutant load allocations for turbidity in
impaired waterbodies, which is the first step toward restoring water quality. TMDLs determine
the pollutant loading a waterbody can assimilate without exceeding the WQS for that pollutant.
TMDLs also establish the pollutant load allocation necessary to meet the WQS established for a
waterbody based on the relationship between pollutant sources and in-stream water quality
conditions. A TMDL consists of a wasteload allocation (WLA), load allocation (LA), and a
margin of safety (MOS). The WLA is the fraction of the total pollutant load apportioned to
point sources, and includes stormwater discharges regulated under the National Pollutant
Discharge Elimination System (NPDES) as point sources. The LA is the fraction of the total
pollutant load apportioned to nonpoint sources. The MOS is a percentage of the TMDL set
aside to account for the lack of knowledge associated with natural process in aquatic systems,
model assumptions, and data limitations.
This report does not stipulate specific control actions (regulatory controls) or management
measures (voluntary best management practices) necessary to reduce turbidity loadings within
each watershed. Watershed-specific control actions and management measures will be
identified, selected, and implemented under a separate process involving stakeholders who live
and work in the watershed; tribe;, and local, state, and federal government agencies.
This TMDL report focuses on waterbodies that ODEQ placed in Category 5 [303(d) list] of
the Water Quality in Oklahoma, 2008 Integrated Report (2008 Integrated Report) for the
beneficial use category Fish and Wildlife Propagation for Sulphur Creek
OK410600010030_00. Figure 1-1 is a location map showing the impaired segment of this
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Oklahoma waterbody and its contributing watershed. This map also displays the locations of
the water quality monitoring (WQM) stations used as the basis for placement of this waterbody
on the Oklahoma 303(d) list. The waterbody and its surrounding watershed are hereinafter
referred to as the Study Area.
The TMDL established in this report is a necessary step in the process to develop the
turbidity controls needed to restore the fish and wildlife propagation designated for the
waterbody. Table 1-1 provides a description of the locations of the WQM stations on the
303(d)-listed waterbody.
Table 1-1 Water Quality Monitoring Stations used for 2008 303(d) Listing Decision
WQM Station
WQM Station
Location
Description
WQM Station
Location Legal
Descriptions
Latitude Longitude
OK410600010030G Sulphur Creek
SW¼ SW¼ NE¼
Section 16-7S-12E
33.94658 -96.049
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Figure 1-1 Watersheds Not Supporting Fish and Wildlife Propagation Use within the
Study Area
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1.2 Watershed Description
General. The Blue River Basin is located in the southern portion of Oklahoma. The
waterbody addressed in this report is located in Bryan County. The Study Area is located in the
South Central Plains ecoregion of Oklahoma.
Table 1-2, derived from the 2000 U.S. Census, shows that Bryan County where this
watershed is located is sparsely populated (U.S. Census Bureau 2000).
Table 1-2 County Population and Density
County Name
Population
(2000 Census)
Population Density
(per square mile)
Bryan 36,534 40
Climate. Table 1-3 summarizes the average annual precipitation for the waterbody.
Average annual precipitation for the waterbody between 1971 and 2000 was 46.4 inches
(Oklahoma Climate Survey 2005).
Table 1-3 Average Annual Precipitation by Watershed
Sulphur Creek Precipitation Summary
Waterbody Name Waterbody ID
Average
Annual
(inches)
Sulphur Creek OK410600010030_00 46.4
Land Use. Table 1-4 summarizes the acreages and the corresponding percentages of the
land use categories for the contributing watershed associated with the Sulphur Creek
watershed. The land use/land cover data were derived from the U.S. Geological Survey
(USGS) 2001 National Land Cover Dataset (USGS 2007). The land use categories are
displayed in Figure 1-2.
The primary land use category in the Study Area is pasture/hay, which makes up 38
percent of the watershed. The second most common land use within the Study Area is
deciduous forest and grassland at 28 and 27 percent, respectively. Bennington, the only town
within the Sulphur Creek watershed, has an estimated population of 289 (U.S. Census Bureau
2000).
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Table 1-4 Land Use Summaries by Watershed
Landuse Category Sulphur Creek
Waterbody ID OK410600010030_00
Percent Herbaceous Wetlands 0%
Percent Woody Wetlands 0%
Percent Cultivated 1%
Percent Pasture/Hay 38%
Percent Grassland 27%
Percent Shrubland 0%
Percent Mixed Forest 0%
Percent Evergreen Forest 0%
Percent Deciduous Forest 28%
Percent Barren 0%
Percent Developed - High Intensity 0%
Percent Developed - Medium Intensity 0%
Percent Developed - Low Intensity 0%
Percent Developed - Open 5%
Percent Water 1%
Acres Herbaceous Wetlands 5
Acres Woody Wetlands 0
Acres Cultivated 165
Acres Pasture/Hay 7,924
Acres Grassland 5,513
Acres Shrubland 0
Acres Mixed Forest 0
Acres Evergreen Forest 12
Acres Deciduous Forest 5,897
Acres Barren 2
Acres Developed - High Intensity 0
Acres Developed - Medium Intensity 9
Acres Developed - Low Intensity 43
Acres Developed - Open 1,011
Acres Water 112
Total (Acres) 20,693
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Figure 1-2 Land Use Map by Watershed
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1.3 Stream Flow Data
Stream flow characteristics and data are key information when conducting water quality
assessments such as TMDLs. While the USGS operates flow gages throughout Oklahoma,
there is no flow gage located on Sulphur Creek. Some flow measurements were collected at
the same time TSS and turbidity water quality samples were collected at various WQM
stations. These data are included in Appendix A along with turbidity and TSS data.
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SECTION 2
PROBLEM IDENTIFICATION AND WATER QUALITY TARGET
2.1 Oklahoma Water Quality Standards
Title 785 of the Oklahoma Administrative Code authorizes the Oklahoma Water Resources
Board (OWRB) to promulgate Oklahoma’s water quality standards and implementation
procedures (OWRB 2008). The OWRB has statutory authority and responsibility concerning
establishment of state water quality standards, as provided under 82 Oklahoma Statute [O.S.],
§1085.30. This statute authorizes the OWRB to promulgate rules …which establish
classifications of uses of waters of the state, criteria to maintain and protect such
classifications, and other standards or policies pertaining to the quality of such waters. [O.S.
82:1085:30(A)]. Beneficial uses are designated for all waters of the state. Such uses are
protected through restrictions imposed by the antidegradation policy statement, narrative water
quality criteria, and numerical criteria (OWRB 2008). The beneficial uses designated for
Sulphur Creek (OK410600010030_00) include primary body contact recreation, warm water
aquatic community, fish consumption, agriculture and aesthetics. The TMDL in this report
addresses fish and wildlife propagation beneficial use for the subcategory warm water aquatic
community. Table 2-1, an excerpt from Appendix B of the 2008 Integrated Report
(ODEQ 2008), summarizes the warm water aquatic community use attainment status and the
scheduled date for TMDL development established by ODEQ for the impaired waterbody of
the Study Area. The 2008 Integrated Report (ODEQ 2008) identifies the Sulphur Creek as
Priority 2 for TMDL development. Priority 2 waterbodies are targeted for TMDL development
by 2013. The TMDL established in this report is a necessary step in the process to restore the
fish and wildlife propagation designation for this waterbody.
The numeric criteria for turbidity to maintain and protect the use of “Fish and Wildlife
Propagation” from Title 785:45-5-12 (f) (7) is as follows:
(A) Turbidity from other than natural sources shall be restricted to not exceed the following
numerical limits:
4. Cool Water Aquatic Community/Trout Fisheries: 10 NTUs;
5. Lakes: 25 NTU; and
6. Other surface waters: 50 NTUs.
(B) In waters where background turbidity exceeds these values, turbidity from point sources
will be restricted to not exceed ambient levels.
(C) Numerical criteria listed in (A) of this paragraph apply only to seasonal base flow
conditions.
(D) Elevated turbidity levels may be expected during, and for several days after, a runoff event.
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Table 2-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List
Waterbody ID Waterbody Name
Stream Miles
Category
TMDL Date
Priority
Warm Water
Aquatic
Community
OK410600010030_00 Sulphur Creek 14.6 5a 2013 2 N
N = Not Supporting;
5a = TMDL is underway or will be scheduled
Source: 2008 Integrated Report, ODEQ 2008
To implement Oklahoma’s WQS for Fish and Wildlife Propagation, OWRB promulgated
Chapter 46, Implementation of Oklahoma’s Water Quality Standards (OWRB 2008). The
excerpt below from Chapter 46: 785:46-15-5, stipulates how water quality data will be assessed
to determine support of fish and wildlife propagation as well as how the water quality target for
TMDLs will be defined for turbidity.
Assessment of Fish and Wildlife Propagation support
(a) Scope. The provisions of this Section shall be used to determine whether the beneficial
use of Fish and Wildlife Propagation or any subcategory thereof designated in OAC 785:45 for
a waterbody is supported.
(e) Turbidity. The criteria for turbidity stated in 785:45-5-12(f)(7) shall constitute the
screening levels for turbidity. The tests for use support shall follow the default protocol in
785:46-15-4(b).
785:46-15-4. Default protocols
(b) Short term average numerical parameters.
(1) Short term average numerical parameters are based upon exposure periods of less than
seven days. Short term average parameters to which this Section applies include, but are not
limited to, sample standards and turbidity.
(2) A beneficial use shall be deemed to be fully supported for a given parameter whose
criterion is based upon a short term average if 10% or less of the samples for that parameter
exceed the applicable screening level prescribed in this Subchapter.
(3) A beneficial use shall be deemed to be fully supported but threatened if the use is
supported currently but the appropriate state environmental agency determines that available
data indicate that during the next five years the use may become not supported due to
anticipated sources or adverse trends of pollution not prevented or controlled. If data from the
preceding two year period indicate a trend away from impairment, the appropriate agency
shall remove the threatened status.
(4) A beneficial use shall be deemed to be not supported for a given parameter whose
criterion is based upon a short term average if at least 10% of the samples for that parameter
exceed the applicable screening level prescribed in this Subchapter.
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2.2 Problem Identification
Turbidity is a measure of water clarity and is caused by suspended particles in the water
column. Because turbidity cannot be expressed as a mass load, total suspended solids (TSS)
are used as a surrogate in this TMDL. Therefore, both turbidity and TSS data are presented in
this section.
Table 2-2 summarizes water quality data collected from the WQM stations between 1991
and 2007 for turbidity. However, as stipulated in Title 785:45-5-12 (f) (7) (C), numeric criteria
for turbidity only apply under base flow conditions. While the base flow condition is not
specifically defined in the Oklahoma Water Quality Standards, DEQ considers base flow
conditions to be all flows less than the 25% flow exceedance frequency (i.e., the lower 75
percent of flows) which is consistent with the USGS Streamflow Conditions Index (USGS
2009). Therefore, Table 2-3 was prepared to represent the subset of these data for samples
collected during base flow conditions. Water quality samples collected under flow conditions
greater than the 25% flow exceedance frequency were therefore excluded from the data set
used for TMDL analysis. The data in Table 2-3 were used to support the decision to place
Sulphur Creek on the ODEQ 2008 303(d) list (ODEQ 2008) for nonsupport of the Fish and
Wildlife Propagation use based on turbidity levels observed in the waterbody.
Table 2-2 Summary of All Turbidity Samples 1991 - 2007
WQM Station
Number of
Turbidity
Samples
Number of
Samples Exceed 50
Nephelometric
Turbidity Units
(NTU)
Percentage of
Samples
Exceeding
Criterion
Average Turbidity
(NTU) Per WQM
Station
OK410600010030G 22 5 23% 82
Table 2-3 Summary of Turbidity Samples Collected During Base Flow Conditions
1992 - 2007
WQM Station
Number of Turbidity
Samples Collected
During Base Flow
Conditions
Number of
Samples
Exceeding 50 NTU
during Base Flow
Conditions
Percentage of
Samples
Exceeding
Criterion
during Base
Flow
Conditions
Average Turbidity
(NTU) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 19 2 11% 34
Table 2-4 summarizes water quality data collected from the WQM stations between 1991
and 2007 for TSS. Table 2-5 presents a subset of these data for samples collected during base
flow conditions. Water quality data for turbidity and TSS are provided in Appendix A.
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Table 2-4 Summary of All TSS Samples 1991 - 2007
WQM Station
Number of TSS
Samples
Average TSS
(mg/L) Per WQM
Station
OK410600010030G 21 76
Table 2-5 Summary of TSS Samples Collected During Base Flow Conditions
1992 -2007
WQM Station
Number of TSS
Samples Collected
During Base Flow
Conditions
Average TSS
(mg/L) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 18 19
2.3 Water Quality Target
The Code of Federal Regulations (40 CFR §130.7(c)(1)) states that, “TMDLs shall be
established at levels necessary to attain and maintain the applicable narrative and numerical
water quality standards.” An individual water quality target established for turbidity must
demonstrate compliance with the numeric criteria prescribed in the Oklahoma WQS
(OWRB 2008). According to the Oklahoma WQS [785:45-5-12(f)(7)], the turbidity criterion
for streams with warm water aquatic community (WWAC) beneficial use is 50 NTUs (OWRB
2008). The turbidity of 50 NTUs applies only to seasonal base flow conditions. Turbidity
levels are expected to be elevated during, and for several days after, a storm event.
TMDLs for turbidity in streams designated as warm water aquatic community must take
into account that no more than 10 percent of the samples may exceed the numeric criterion of
50 NTU. However, as described above, because turbidity cannot be expressed as a mass load,
TSS is used as a surrogate in this TMDL. Since there is no numeric criterion in the Oklahoma
WQS for TSS, a specific method must be developed to convert the turbidity criterion to TSS
based on a relationship between turbidity and TSS. The method for deriving the relationship
between turbidity and TSS and for calculating a water body specific water quality target using
TSS is summarized in Section 4 of this report.
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SECTION 3
POLLUTANT SOURCE ASSESSMENT
A pollutant source assessment characterizes known and suspected sources of pollutant
loading to impaired waterbodies. Sources within a watershed are categorized and quantified to
the extent that information is available. Turbidity may originate from NPDES-permitted
facilities, fields, construction sites, quarries, stormwater runoff and eroding stream banks.
Point sources are permitted through the NPDES program. NPDES-permitted facilities that
discharge treated wastewater are required to monitor for TSS in accordance with their permit.
Nonpoint sources are diffuse sources that typically cannot be identified as entering a waterbody
through a discrete conveyance at a single location. These sources may involve land activities
that contribute TSS to surface water as a result of rainfall runoff. For the TMDL in this report,
all sources of pollutant loading not regulated by NPDES permits are considered nonpoint
sources.
The 2008 Integrated Water Quality Assessment Report (ODEQ 2008) listed potential
sources of turbidity in Sulphur Creek (OK410600010030_00) as grazing in riparian corridors of
streams and creeks, highway/road/bridge runoff (non-construction related), non-irrigated crop
production, rangeland grazing, and other unknown sources.
3.1 NPDES-Permitted Facilities
Under 40CFR, §122.2, a point source is described as a discernable, confined, and discrete
conveyance from which pollutants are or may be discharged to surface waters. NPDES-permitted
facilities can be characterized as continuous or stormwater related discharges.
NPDES-permitted facilities classified as point sources include:
NPDES municipal wastewater treatment plant (WWTP);
NPDES Industrial WWTP Discharges;
NPDES municipal separate storm sewer discharge (MS4);
NPDES Concentrated Animal Feeding Operation (CAFO);
NPDES multi-sector general permits; and
NPDES construction stormwater discharges.
Continuous point source discharges from municipal and industrial WWTPs, could result in
discharge of elevated concentrations of TSS if a facility is not properly maintained, is of poor
design, or flow rates exceed capacity. However, in most cases suspended solids discharged by
WWTPs consist primarily of organic solids rather than inorganic suspended solids (i.e., soil and
sediment particles from erosion or sediment resuspension). Discharges of organic suspended
solids from WWTPs are addressed by ODEQ through its permitting of point sources to
maintain WQS for dissolved oxygen. and are not considered a potential source of turbidity in
this TMDL report. Discharges of TSS will be considered to be organic suspended solid if the
discharge permit includes a limit for BOD or CBOD. Only WWTP discharges of inorganic
suspended solids will be considered and will receive wasteload allocations.
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Stormwater runoff from MS4 areas, facilities under multi-sector general permits, and
NPDES construction stormwater discharges, which are regulated under the USEPA NPDES
Program, can contain TSS concentrations. 40 C.F.R. § 130.2(h) requires that NPDES-regulated
storm water discharges must be addressed by the wasteload allocation component of a TMDL.
However, any stormwater discharge by definition occurs during or immediately following
periods of rainfall and elevated flow conditions when where Oklahoma Water Quality Standard
for turbidity does not apply. Oklahoma Water Quality Standards specify that the criteria for
turbidity “apply only to seasonal base flow conditions” and go on to say “Elevated turbidity
levels may be expected during, and for several days after, a runoff event” [OAC 785:45-5-
12(f)(7)]. In other words, the turbidity impairment status is limited to base flow conditions and
stormwater discharges from MS4 areas or construction sites do not contribute to the violation
of Oklahoma’s turbidity standard. Therefore, WLA for NPDES-regulated storm water
discharges is essentially considered unnecessary in this TMDL report and will not be included
in the TMDL calculations.
3.1.1 Continuous Point Source Discharges
There are no municipal or industrial NPDES-permitted facilities within the Study Area.
3.1.2 Concentrated Animal Feeding Operations
There are no CAFOs within the Study Area.
3.1.3 Stormwater Permits for MA4 and Construction Activities
There are no urbanized areas designated as MS4s within this Study Area. A general
stormwater permit is required for construction activities. Permittees are authorized to discharge
pollutants in stormwater runoff associated with construction activities for construction sites.
Stormwater discharges occur only during or immediately following periods of rainfall and
elevated flow conditions when the turbidity criteria do not apply and are not considered
potential contributors to turbidity impairment.
3.1.4 Section 404 permits
Section 404 of the Clean Water Act (CWA) establishes a program to regulate the discharge
of dredged or fill material into waters of the United States, including wetlands. Activities in
waters of the United States regulated under this program include fill for development, water
resource projects (such as dams and levees), infrastructure development (such as highways and
airports) and mining projects. Section 404 requires a permit before dredged or fill material may
be discharged into waters of the United States, unless the activity is exempt from Section 404
regulation (e.g. certain farming and forestry activities).
Section 404 permits are administrated by the U.S. Army Corps of Engineers. EPA reviews
and provides comments on each permit application to make sure it adequately protects water
quality and complies with applicable guidelines. Both USACE and EPA can take enforcement
actions for violations of Section 404.
Discharge of dredged or fill material in waters can be a significant source of turbidity/TSS.
The federal Clean Water Act requires that a permit be issued for activities which discharge
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dredged or fill materials into the waters of the United States, including wetlands. The state of
Oklahoma will use its Section 401 certification authority to ensure Section 404 permits protect
oklahoma water quality standards.
3.2 Nonpoint Sources
Nonpoint sources include those sources that cannot be identified as entering the waterbody
at a specific location. The relatively homogeneous land use/land cover categories within the
Study Area are associated with agricultural and range management activities. This suggests
that potential nonpoint sources of TSS include sediments originating from grazing in riparian
corridors of streams and creeks, highway/road/bridge runoff (non-construction related), non-irrigated
crop production, rangeland grazing and other sources of sediment loading
(ODEQ 2008). Elevated turbidity measurements can be caused by stream bank erosion
processes, stormwater runoff events and channel disturbances. However, there is insufficient
data available to quantify contributions of TSS from these processes. TSS or sediment loading
can also occur under non-runoff conditions as a result of anthropogenic activities in riparian
corridors which cause erosive conditions. Sediment loading of streams can also originate from
natural erosion processes, including the weathering of soil, rocks, and uncultivated land;
geological abrasion; and other natural phenomena. Given the lack of data to establish the
background conditions for TSS/turbidity, separating background loading from nonpoint sources
is not feasible in this TMDL development.
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Figure 3-1 Locations of Permitted Facilities in the Study Area
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SECTION 4
TECHNICAL APPROACH AND METHODS
The objective of a TMDL is to estimate allowable pollutant loads and to allocate these
loads to the known pollutant sources in the watershed so appropriate control measures can be
implemented and the WQS achieved. A TMDL is expressed as the sum of three elements as
described in the following mathematical equation:
TMDL = Σ WLA + Σ LA + MOS
The WLA is the portion of the TMDL allocated to existing and future point sources. The
LA is the portion of the TMDL allocated to nonpoint sources, including natural background
sources. The MOS is intended to ensure that WQS will be met. Thus, the allowable pollutant
load that can be allocated to point and nonpoint sources can then be defined as the TMDL
minus the MOS.
4.1 Determining a Surrogate Target
40 CFR, §130.2(1), states that TMDLs can be expressed in terms of mass per time,
toxicity, or other appropriate measures. Turbidity is a commonly measured indicator of the
suspended solids load in streams. However, turbidity is an optical property of water, and
measures scattering of light by suspended solids and colloidal matter. To develop TMDLs, a
gravimetric (mass-based) measure of solids loading is required to express loads. There is often
a strong relationship between the total suspended solids concentration and turbidity. Therefore,
the TSS load, which is expressed as mass per time, is used as a surrogate for turbidity and
represents the maximum one-day load the stream can assimilate while still attaining the WQS.
To determine the relationship between turbidity and TSS, a linear regression between TSS
and turbidity was developed using data collected from 1991 to 2007 at one station within the
Study Area. Prior to developing the regression the following steps were taken to refine the
dataset:
Assign values to censored data (i.e., measured concentrations lower than the analytical
quantitation limit and, thus, reported as less than the quantitation limit). For example,
using 9.99 to replace samples reported as “<10”;
Check rainfall data on the day when samples were collected and on the previous two
days. If there was a significant rainfall event (>= 1.0 inch) in any of these days, the
sample will be excluded from regression analysis with one exception. If the significant
rainfall happened on the sampling day and the turbidity reading was less than 25 NTUs
(half of turbidity standard for streams), the sample will not be excluded from analysis
because most likely the rainfall occurred after the sample was taken;
Remove data collected under high flow conditions exceeding the base-flow criterion.
This means that measurements corresponding to flow exceedance frequencies lower
than 25% were not used in the regression; and
Log-transform both turbidity and TSS data to minimize effects of their non-linear data
distributions.
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When ordinary least squares regression (OLS) is applied to ascertain the best relationship
between two variables (i.e., X and Y), one variable (Y) is considered “dependent” on the other
variable (X), but X must be considered “independent” of the other, and known without
measurement error. OLS minimizes the differences, or residuals, between measured Y values
and Y values predicted based on the X variable.
For current purposes, a relationship is necessary to predict TSS concentrations from
measured turbidity values, but also to translate the TSS-based TMDL back to in-stream
turbidity values. For this purpose, an alternate regression fitting procedure known as the line of
organic correlation (LOC) was applied. The LOC has three advantages over OLS (Helsel and
Hirsch 2002):
LOC minimizes fitted residuals in both the X and Y directions;
It provides a unique best-fit line regardless of which parameter is used as the
independent variable; and
Regression-fitted values have the same variance as the original data.
The LOC minimizes the areas of the right triangles formed by horizontal and vertical lines
drawn from observations to the fitted line. The slope of the LOC line equals the geometric
mean of the Y on X (TSS on turbidity) and X on Y (turbidity on TSS) OLS slopes, and is
calculated as:
x
y
s
s
m1 m m' sign[r]
where m1 is the slope of the LOC line, m is the TSS on turbidity OLS slope, m’ is the turbidity
on TSS OLS slope, r is the TSS-turbidity correlation coefficient, sy is the standard deviation of
the TSS measurements, and sx is the standard deviation of the turbidity measurements.
The intercept of the LOC (b1) is subsequently found by fitting the line with the LOC slope
through the point (mean turbidity, mean TSS). The correlation between TSS and turbidity,
along with the LOC and the OLS lines are shown in Figure 4-1.
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Figure 4-1 Linear Regression for TSS-Turbidity under Base-flow Conditions for
Sulphur Creek (OK 410600010030_00)
The normalized root mean square error (NRMSE) and R-square (R2) were used as the
primary measures of goodness-of-fit. As shown in Figure 4-1, the LOC yields a NRMSE value
of 14.9 which means the root mean square error (RMSE) is 14.9% of the average of the
measured TSS values. The R-square (R2) value indicates the fraction of the total variance in
TSS or turbidity observations that is explained by the LOC.
It was noted that there were a few outliers that exerted undue influence on the regression
relationship. These outliers were identified by applying the Tukey’s Boxplot method
(Tukey 1977) to the dataset of the distances from observed points to the regression line. The
Tukey Method is based on the interquartile range (IQR), the difference between the 75th
percentile (Q3) and 25th percentile (Q1) of distances between observed points and the LOC.
Using the Tukey method, any point with an error greater than Q3 + 1.5* IQR or less than Q1 –
1.5*IQR was identified as an outlier and removed from the regression dataset. The above
regressions were calculated using the dataset with outliers removed.
The Tukey Method is equivalent to using three times the standard deviation to identify
outliers if the residuals (observed - predicted) follow a normal distribution. The probability of
sampling results being within three standard deviations of the mean is 99.73% while the
probability for the Tukey Method is 99.65%. If three times the standard deviation is used to
identify outliers, it is necessary to first confirm that the residuals are indeed normally
distributed. This is difficult to do because of the size limitations of the existing turbidity &
TSS dataset. Tukey’s method does not rely on any assumption about the distribution of the
residuals. It can be used regardless of the shape of distribution.
Using the regression equation shown in Figure 4-1, a turbidity value of 50 NTU (standard
applicable to Sulphur Creek) corresponds to a TSS concentration of 31.4 mg/L.
1
10
100
1000
1 10 100 1000
TSS (mg/L)
Turbidity (NTU)
log(TSS) = 0.7342*log(Turb) +
0.2489
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4.2 Using Load Duration Curves to Develop TMDLs
The TMDL calculations presented in this report are derived from load duration curves
(LDC). LDCs facilitate rapid development of TMDLs, and as a TMDL development tool,
indicate whether impairments are associated with point or nonpoint sources. The technical
approach for using LDCs for TMDL development includes the four following steps described
in Subsections 4.3 through 4.4 below:
Preparing flow duration curves for gaged and ungaged WQM stations;
Estimating loading in the receiving water using measured TSS water quality data and
turbidity-converted data;
Determining the overall percent reduction goal (PRG) necessary to attain WQS; and
Historically, in developing WLAs for pollutants from point sources, it was customary to
designate a critical low flow condition (e.g., 7Q2) at which the maximum permissible loading
was calculated. As water quality management efforts expanded in scope to quantitatively
address nonpoint sources of pollution and various types of pollutants, it became clear that this
single critical low flow condition was inadequate to ensure adequate water quality across a
range of flow conditions. Use of the LDC obviates the need to determine a design storm or
selected flow recurrence interval with which to characterize the appropriate flow level for the
assessment of critical conditions. For waterbodies impacted by both point and nonpoint
sources, the “nonpoint source critical condition” would typically occur during high flows, when
rainfall runoff would contribute the bulk of the pollutant load, while the “point source critical
condition” would typically occur during low flows, when point source discharges would
dominate the base flow of the impaired water. However, flow range is only a general indicator
of the relative proportion of point/nonpoint contributions. It is not used in this report to
quantify point source or nonpoint source contributions. Violations that occur during low flows
may not be caused exclusively by point sources. Violations have been noted in some
watersheds that contain no point sources.
LDCs display the maximum allowable load over the complete range of flow conditions by
a line using the calculation of flow multiplied by the water quality criterion. The TMDL can be
expressed as a continuous function of flow, equal to the line, or as a discrete value derived from
a specific flow condition.
4.3 Development of Flow Duration Curves
Flow duration curves serve as the foundation of LDCs and are graphical representations of
the flow characteristics of a stream at a given site. Flow duration curves utilize the historical
hydrologic record from stream gages to forecast future recurrence frequencies. Many WQM
stations throughout Oklahoma do not have long-term flow data; therefore, flow frequencies
must be estimated. The most basic method to estimate flows at an ungaged site involves
1) identifying a downstream flow gage; 2) calculating the contributing drainage areas of the
ungaged sites and the flow gage; and 3) calculating daily flows at the ungaged site by using the
flow at the gaged site multiplied by the drainage area ratio. A more complex approach used to
support this analysis also considers watershed differences in rainfall, land use, and the
hydrologic properties of soil that govern runoff and retention. For the Sulphur Creek
watershed, flows were projected using data from USGS 07332500, Blue River near Blue, OK.
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A more detailed explanation of the methods for estimating flow at ungaged WQM stations is
provided in Appendix B.
Flow duration curves are a type of cumulative distribution function. The flow duration
curve represents the fraction of flow observations that equal or exceed a given flow at the site
of interest. The observed flow values are first ranked from highest to lowest then, for each
observation, the percentage of observations equal to or exceeding that flow is calculated. The
flow value is read from the ordinate (y-axis), which is typically on a logarithmic scale since the
high flows would otherwise overwhelm the low flows. The flow exceedance frequency is read
from the abscissa (x-axis), which is numbered from 0 to 100 percent, and may or may not be
logarithmic. The flow exceedance frequency is defined as percent of time a given flow was
equaled or exceeded based on daily flow values. Therefore, the lowest measured flow occurs at
an exceedance frequency of 100 percent indicating that flow has equaled or exceeded this value
100 percent of the time, while the highest measured flow is found at an exceedance frequency
of 0 percent. The median flow occurs at a flow exceedance frequency of 50 percent. The flow
exceedance frequencies of the USGS gage used to project flows for this report are provided in
Appendix B.
While the number of observations required to develop a flow duration curve is not
rigorously specified, a flow duration curve is usually based on more than 1 year of
observations, and encompasses inter-annual and seasonal variation. Ideally, the drought of
record and flood of record are included in the observations. For this purpose, the long-term
flow gaging stations operated by the USGS are utilized (USGS 2007a).
A typical semi-log flow duration curve exhibits a sigmoidal shape, bending upward near a
flow exceedance frequency value of 0 percent and downward at a frequency near 100 percent,
often with a relatively constant slope in between. For sites that on occasion exhibit no flow, the
curve will intersect the x-axis at a frequency less than 100 percent. As the number of
observations at a site increases, the line of the LDC tends to appear smoother. However, at
extreme low and high flow values, flow duration curves may exhibit a “stair step” effect due to
the USGS flow data rounding conventions near the limits of quantitation.
Flow duration curves are generated using a DEQ automated application referred to as the
Oklahoma TMDL toolbox. Figure 4-2 shows the flow duration curve generated from the
Oklahoma TMDL toolbox for Sulphur Creek using flow data from 1984 to 2006. The USGS
National Water Information System serves as the primary source of flow measurements for the
application. All available daily average flow values for all gages in Oklahoma, as well as the
nearest upstream and downstream gages in adjacent states, were retrieved for use in the
application. The application includes a data update module that automatically downloads the
most recent USGS data and appends it to the existing flow database.
Some instantaneous flow measurements were available from various agencies. These were
not combined with the daily average flows or used in calculating flow percentiles, but were
matched to TSS and/or turbidity grab measurements collected at the same site and time. When
available, these instantaneous flow measurements were used in lieu of the daily average flow to
calculate instantaneous TSS loads.
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Figure 4-2 Flow Duration Curve for Sulphur Creek (OK410600010030_00)
4.4 Development of TMDLs Using Load Duration Curves
The final step in the TMDL calculation process involves a group of additional
computations derived from the preparation of LDCs. These computations are necessary to
derive a PRG (which is one method of presenting how much TSS loading must be reduced to
meet turbidity WQS in the impaired watershed).
Step 1: Generate LDCs. LDCs are similar in appearance to flow duration curves;
however, the ordinate is expressed in terms of a load typically in lbs/day. The curve represents
the water quality target for TSS (31 mg/L) expressed in terms of a load through multiplication
by the continuum of flows historically observed at this site. The basic steps to generating an
LDC involve:
obtaining daily flow data for the site of interest from the USGS (or project flow using
Oklahoma TMDL Toolbox if station is ungaged);
sorting the flow data and calculating flow exceedance frequencies for the time period
and season of interest;
obtaining available turbidity and TSS water quality data;
matching the water quality observations with the flow data from the same date;
displaying a curve on a plot that represents the allowable load multiplying the actual or
estimated flow by the WQtarget for TSS;
multiplying the flow by the water quality parameter concentration to calculate daily
loads (for sampling events with both TSS and turbidity data, the measured TSS value
is used; if only turbidity was measured, the value was converted to TSS using the
regression equation in Figure 4-1); then
0.1
1.0
10.0
100.0
1000.0
10000.0
0 10 20 30 40 50 60 70 80 90 100
Flow (cfs)
Flow Exceedence Frequency (%)
Flow Duration Curver (OK410600010030_00)
High flow conditions
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plotting the flow exceedance frequencies and daily load observations in a load duration
plot.
The culmination of these steps is expressed in the following formula, which is displayed on
the LDC as the TMDL curve:
TMDL (lb/day) = WQtarget * flow (cfs) * unit conversion factor
where: WQtarget = 31.4 mg/L (TSS)
unit conversion factor = 5.39377 L*s*lb /(ft3*day*mg)
The flow exceedance frequency (x-value of each point) is obtained by looking up the
historical exceedance frequency of the measured or estimated flow; in other words, the percent
of historical observations that equal or exceed the measured or estimated flow. Historical
observations of TSS and/or turbidity concentrations are paired with flow data and are plotted on
the LDC. The TSS load (or the y-value of each point) is calculated by multiplying the TSS
concentration (measured or converted from turbidity) (mg/L) by the instantaneous flow (cfs) at
the same site and time, with appropriate volumetric and time unit conversions. TSS loads
representing exceedance of water quality criteria fall above the TMDL line.
As noted earlier, runoff has a strong influence on loading of nonpoint source pollution yet
flows do not always correspond directly to local runoff. High flows may occur in dry weather
due to upstream precipitation events or releases form upstream dams. Runoff influence may be
observed with low or moderate flows depending on antecedent conditions.
Step 2: Define MOS. The MOS may be defined explicitly or implicitly. A typical
explicit approach would reserve some specific fraction of the TMDL as the MOS. In an
implicit approach, conservative assumptions used in developing the TMDL are relied upon to
provide an MOS to assure that WQSs are attained. For turbidity (TSS) TMDLs an explicit
MOS is derived from the NRMSE established by the turbidity/TSS regression analysis
conducted for each waterbody. This approach for setting an explicit MOS has been used in
other approved turbidity TMDLs
For the TMDLs in this report, an explicit MOS of 10 percent was selected.
Step 3: Calculate WLA. As previously stated, the pollutant load allocation for point
sources is defined by the WLA. For TMDL development purposes when addressing turbidity
or TSS, a WLA will be established for wastewater (continuous) discharges in impaired
watersheds that do not have a BOD or CBOD permit limit but do have a TSS limit. These point
source discharges of inorganic suspended solids will be assigned a TSS WLA as part of
turbidity TMDLs to ensure WQS can be maintained.
The LDC approach recognizes that the assimilative capacity of a waterbody depends on the
flow, and that maximum allowable loading will vary with flow condition. TMDLs can be
expressed in terms of maximum allowable concentrations, or as different maximum loads
allowable under different flow conditions, rather than single maximum load values. A load-based
approach meets the requirements of 40 CFR, 130.2(i) for expressing TMDLs “in terms of
mass per time, toxicity, or other appropriate measures.”
WLA for WWTP. WLAs may be set to zero for watersheds with no existing or planned
continuous permitted point sources such as Sulphur Creek.
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WLA for Permitted Stormwater. For turbidity TMDLs, WLAs for permitted stormwater
such as MS4s, construction, and multi-sector general permits are not calculated since these
discharges occur under high flow conditions when the turbidity criteria do not apply.
Step 4: Calculate LA. Given the lack of data and the variability of storm events, it is
difficult to quantify discharges that accurately represent projected loadings from nonpoint
sources. LAs can be calculated under different flow conditions as the water quality target load
minus the WLA. The LA is represented by the area under the LDC but above the WLA. The
LA at any particular flow exceedance is calculated as shown in the equation below.
LA = TMDL - WLA - MOS
Step 5: Estimate LA Load Reduction. After existing loading estimates are computed,
nonpoint load reduction estimates are calculated by using the difference between estimated
existing loading and the allowable load expressed by the LDC (TMDL-MOS). This difference
is expressed as the overall PRG for the impaired waterbody. For turbidity, the PRG is the load
reduction that ensures that no more than 10 percent of the samples under flow-base conditions
exceed the TMDL.
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SECTION 5
TMDL CALCULATIONS
5.1 Estimated Loading and Critical Conditions
USEPA regulations at 40 CFR 130.7(c) (1) require TMDLs to take into account critical
conditions for stream flow, loading, and all applicable water quality standards. To accomplish
this, available instream WQM data were evaluated with respect to flows and magnitude of
water quality criteria exceedance using LDCs.
To calculate the TSS load at the WQtarget, the flow rate at each flow exceedance frequency
is multiplied by a unit conversion factor (5.39377 L*s*lb /ft3/day/mg) and the TSS target
(31 mg/L). This calculation produces the maximum TSS load in the stream that will result in
attainment of the 50 NTU standard for turbidity. The allowable TSS loads at the WQS
establish the TMDL and are plotted versus flow exceedance frequency as a LDC. The x-axis
indicates the flow exceedance frequency, while the y-axis is expressed in terms of a TSS load
in pounds per day.
To estimate existing loading, TSS and turbidity observations from 1991 to 2007 are paired
with the flows measured or estimated in that segment on the same date. For sampling events
with both TSS and turbidity data, the measured TSS value is used; if only turbidity was
measured, the value was converted to TSS using the regression equation in Figure 4-1.
Pollutant loads are then calculated by multiplying the TSS concentration by the flow rate and
the unit conversion factor. The associated flow exceedance frequency is then matched with the
measured flow from the tables provided in Appendix B. The observed TSS or converted
turbidity loads are then added to the LDC plot as points. These points represent individual
ambient water quality samples of TSS. Points above the LDC indicate the TSS target was
exceeded at the time of sampling. Conversely, points under the LDC indicate the sample did
not exceed the WQtarget. Figure 5-1 shows the LDC developed for Sulphur Creek. It is noted
that the LDC plot includes data under all flow conditions to show the overall condition of the
stream. However, it is noted that the turbidity standard only applies for base-flow conditions.
Thus, when assessing beneficial use assessment, only the portion of the graph corresponding to
flows from the 25% to 100% flow exceedance frequency should be used.
The LDC approach recognizes that the assimilative capacity of a waterbody depends on the
flow, and that maximum allowable loading varies with flow condition. Existing loading, and
load reductions required to meet the TMDL water quality target can also be calculated under
different flow conditions. The difference between existing loading and the water quality target
is used to calculate the loading reductions required. The overall PRG is calculated for Sulphur
Creek as the reduction in load required so no more than 10 percent of the samples collected
under base-flow conditions would exceed 28.3 mg/L (90 percent of the TSS WQtarget to account
for the explicit MOS). This is done through an iterative process of taking a series of percent
reduction values applying each value uniformly between the concentrations of samples and
verifying that no more than 10 percent of the samples exceed the water quality target
concentration. The concentrations are derived from only those samples after high flow samples
are excluded. The PRG for Sulphur Creek is estimated to be 11.7 percent.
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Figure 5-1 Load Duration Curve for Total Suspended Solids in Sulphur Creek
(OK410600010030_00)
As shown in Figure 5-1, TSS levels exceed the water quality target less than 20% of the
time.
5.2 Wasteload Allocation
The WLA_WWTF for the Study Area is zero.
No wasteload allocations are needed for stormwater dischargers. By definition, any
stormwater discharge occurs during periods of rainfall and elevated flow conditions.
Oklahoma’s Water Quality Standards specify that the criteria for turbidity “apply only to
seasonal base flow conditions” and go on to say “Elevated turbidity levels may be expected
during, and for several days after, a runoff event” [OAC 785:45-5-12(f)(7)]. Therefore, WLA
for NPDES-regulated storm water discharges is essentially considered unnecessary in this
TMDL report and will not be included in the TMDL calculations. Conditions in existing
stormwater permits are sufficient to protect receiving waters.
To accommodate the potential for future growth in the watershed, 1% of TSS loading is
reserved as part of the WLA.
5.2.1 Section 404 permits
No TSS wasteload allocations were set aside for Section 404 permits. The state will use its
Section 401 certification authority to ensure Section 404 permits protect Oklahoma water
quality standards and comply with TSS TMDLs in this report. Section 404 permits will be
conditioned to meet one of the following two conditions to be certified by the state:
1.0
10.0
100.0
1000.0
10000.0
100000.0
1000000.0
10000000.0
0 10 20 30 40 50 60 70 80 90 100
TSS Load (lbs/day)
Flow Exceedance Frequency (%)
Load Duration Curver (OK410600010030_00)
High flow conditions
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Include TSS limits in the permit and establish a monitoring requirement to ensure
compliance with turbidity standard and TSS TMDLs.
Submit to the ODEQ a BMP turbidity reduction plan which should include all
practicable turbidity control techniques. The turbidity reduction plan must be approved
first before a Section 404 permit can be issued.
5.3 Load Allocation
As discussed in Section 3.2, pollutant loading to the receiving streams of each waterbody
emanate from a number of different nonpoint sources. The data analysis and the LDCs
demonstrate that exceedances of the turbidity WQS at the WQM stations are the result of a
variety of nonpoint sources. The LA is calculated as the difference between the TMDL, MOS,
and WLA as follows:
LA = TMDL – WLA_WWTP – WLA_growth - MOS
5.4 Seasonal Variability
Federal regulations (40 CFR §130.7(c)(1)) require that TMDLs account for seasonal
variation in watershed conditions and pollutant loading. The TMDL established in this report
adhere to the seasonal application of the Oklahoma WQS for turbidity, which applies to
seasonal base flow conditions only. Seasonal variation was also accounted for in this TMDL
by using more than 5 years of water quality data and by using the longest period of USGS flow
records possible when estimating flows to develop flow exceedance frequency.
5.5 Margin of Safety
Federal regulations (40 CFR §130.7(c)(1)) require that TMDLs include an MOS. The
MOS is a conservative measure incorporated into the TMDL equation that accounts for the lack
of knowledge associated with calculating the allowable pollutant loading to ensure WQSs are
attained. USEPA guidance allows for use of implicit or explicit expressions of the MOS, or
both. When conservative assumptions are used in development of the TMDL, or conservative
factors are used in the calculations, the MOS is implicit. When a specific percentage of the
TMDL is set aside to account for lack of knowledge, then the MOS is considered explicit.
An explicit Margin of Safety of 10% was selected in this TMDL report.
5.6 TMDL Calculations
This TMDL was derived using the LDC method. A TMDL is expressed as the sum of all
WLAs (point source loads), LAs (nonpoint source loads), and an appropriate MOS, which
attempts to account for lack of knowledge concerning the relationship between effluent
limitations and water quality.
This definition can be expressed by the following equation:
TMDL = Σ WLA + Σ LA + MOS
The TMDL represents a continuum of desired load over all flow conditions, rather than
fixed at a single value, because loading capacity varies as a function of the flow present in the
stream. The higher the flow is, the more wasteload the stream can handle without violating
water quality standards.
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Regardless of the magnitude of the WLA calculated in these TMDLs, future new
discharges or increased load from existing discharges will be considered consistent with the
TMDL provided the NPDES permit requires instream criteria to be met.
TheTMDL, WLA, LA, and MOS are calculated at every 5th flow interval percentile
(Table 5-1).
Table 5-1 Turbidity TMDL based on Total Suspended Solids Calculations for
Sulphur Creek (OK410600010030_00)
Percentile
Flow
(cfs)
TMDL
(lb/day)
WLA (lb/day) LA
(lb/day)
MOS
WWTP MS4 Growth (lb/day)
0 1,668 NA 0 0 NA NA NA
5 45 NA 0 0 NA NA NA
10 19 NA 0 0 NA NA NA
15 12 NA 0 0 NA NA NA
20 9.2 NA 0 0 NA NA NA
25 7.4 1,253 0 0 12.5 1,115 125
30 6 1,016 0 0 10.2 904 102
35 5 847 0 0 8.5 754 85
40 4.2 711 0 0 7.1 633 71
45 3.7 627 0 0 6.3 558 63
50 3.2 542 0 0 5.4 482 54
55 2.7 457 0 0 4.6 407 46
60 2.3 390 0 0 3.9 347 39
65 2.1 356 0 0 3.6 317 36
70 1.8 305 0 0 3.0 271 30
75 1.6 271 0 0 2.7 241 27
80 1.4 237 0 0 2.4 211 24
85 1.2 203 0 0 2.0 181 20
90 1 169 0 0 1.7 151 17
95 0.7 119 0 0 1.2 106 12
100 0 0 0 0 0 0 0
5.7 Reasonable Assurances
ODEQ will collaborate with a host of other state agencies and local governments working
within the boundaries of state and local regulations to target available funding and technical
assistance to support implementation of pollution controls and management measures. Various
water quality management programs and funding sources provide a reasonable assurance that
the pollutant reductions as required by this TMDL can be achieved and water quality can be
restored to maintain designated uses. ODEQ’s Continuing Planning Process (CPP), required by
the CWA §303(e)(3) and 40 CFR 130.5, summarizes Oklahoma’s commitments and programs
aimed at restoring and protecting water quality throughout the state (ODEQ 2006). The CPP
can be viewed from ODEQ’s website at 2006 Continuing Planning Process. Table 5-2 provides
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a partial list of the state partner agencies ODEQ will collaborate with to address point and
nonpoint source reduction goals established by TMDLs.
Table 5-2 Partial List of Oklahoma Water Quality Management Agencies
Agency Web Link
Oklahoma Conservation Commission www.conservation.ok.gov
Oklahoma Department of Wildlife
Conservation
http://www.wildlifedepartment.com/watchabl.htm
Oklahoma Department of Agriculture,
Food, and Forestry
http://www.oda.state.ok.us/aems.htm
Oklahoma Water Resources Board http://www.owrb.ok.gov
Nonpoint source pollution in Oklahoma is managed by the Oklahoma Conservation
Commission (OCC). The OCC works with state partners such as Oklahoma Department of
Agriculture, Food, and Forestry (ODAFF) and federal partners such USEPA and the National
Resources Conservation Service (NRCS), to address water quality problems similar to those
seen in the Study Area. The primary mechanisms used for management of nonpoint source
pollution are incentive-based programs that support the installation of BMPs and public
education and outreach. Other programs include regulations and permits for CAFOs. The
CAFO Act, as administered by the ODAFF, provides CAFO operators the necessary tools and
information to deal with the manure and wastewater animals produce so streams, lakes, ponds,
and groundwater sources are not polluted.
As authorized by Section 402 of the CWA, the ODEQ has delegation of the NPDES
Program in Oklahoma, except for certain jurisdictional areas related to agriculture and the oil
and gas industry retained by State Department of Agriculture and Oklahoma Corporation
Commission, for which the USEPA has retained permitting authority. The NPDES Program in
Oklahoma is implemented via Title 252, Chapter 606 of the Oklahoma Pollution Discharge
Elimination System (OPDES) Act and in accordance with the agreement between ODEQ and
USEPA relating to administration and enforcement of the delegated NPDES Program.
Implementation of point source WLAs is done through permits issued under the OPDES
program.
The reduction rate called for in this TMDL report is 11.7 percent.
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SECTION 6
PUBLIC PARTICIPATION
This report was submitted to EPA for technical review and was technically accepted on
July 01, 2010. A public notice was circulated on July 15, 2010 to local newspapers and/or
other publications in the area affected by this TMDL and persons on the DEQ contact list. The
public comment period ended on August 30, 2010. No requests for a public meeting were
received. No comments were received.
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SECTION 7
REFERENCES
Helsel, D.R. and R.M. Hirsch 2002. Statistical Methods in Water Resources. U.S. Department of the
Interior, U.S. Geological Survey, September 2002.
ODEQ 2006. Continuing Planning Process. 2006 Edition.
ODEQ 2008. Water Quality in Oklahoma, 2008 Integrated Report. 2008
Oklahoma Climate Survey. 2005. Viewed March 6, 2009 in
http://climate.ocs.ou.edu/county_climate/Products/County_Climatologies/
OWRB. 2008. Oklahoma Water Resources Board. 2008 Water Quality Standards.
Tukey, J.W. 1977. Exploratory Data Analysis. Addison-Wesely.
U.S. Census Bureau 2000. http://www.census.gov/main/www/cen2000.html
USEPA 1991. Guidance for Water Quality-Based Decisions: The TMDL Process. Office of Water,
USEPA 440/4-91-001.
USEPA 2003. Guidance for 2004 Assessment, Listing and Reporting Requirements Pursuant to Sections
303(d) and 305(b) of the Clean Water Act, TMDL -01-03 - Diane Regas-- July 21, 2003.
USGS 2007. Multi-Resolution Land Characteristics Consortium. http://www.mrlc.gov/index.asp
USGS 2007a. USGS Daily Streamflow Data. http://waterdata.usgs.gov/nwis/sw
USGS 2009. USGS National Water Information System Website.
http://waterdata.usgs.gov/nwis/?percentile_help
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APPENDIX A
AMBIENT WATER QUALITY DATA
1991 - 2007
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Appendix A
Ambient Water Quality Data
1991 - 2007
WQM Station Date
Turbidity
(NTU)
Total
Suspended
Solids
(mg/L)
Flow
(cfs)
Flow
condition1
410600-01-0030G 9/24/1991 7 15
410600-01-0030G 4/15/1992 5.29 42
410600-01-0030G 10/15/1992 6.29 12
410600-01-0030G 8/4/1993 8 6
410600-01-0030G 6/21/2005 8.01 <10 0.188
410600-01-0030G 7/20/2005 3.56 0.033
410600-01-0030G 7/26/2005 12.7 17
410600-01-0030G 10/11/2005 34.4 27
410600-01-0030G 11/8/2005 31.4 32
410600-01-0030G 12/13/2005 13.5 12
410600-01-0030G 1/24/2006 13.5 <10 Rainfall event
410600-01-0030G 2/28/2006 208 108 0.216
410600-01-0030G 4/4/2006 21.9 15 0.378
410600-01-0030G 5/9/2006 36.3 <10 0.353
410600-01-0030G 6/20/2006 150 <10
410600-01-0030G 10/2/2006 41.5 16
410600-01-0030G 11/6/2006 240 79 20.742 High flow
410600-01-0030G 12/12/2006 19.7 <10 0.511
410600-01-0030G 1/22/2007 55.8 11 21 High flow
410600-01-0030G 2/20/2007 4.64 <10 1.754
410600-01-0030G 3/26/2007 16.8 <10 0.779
410600-01-0030G 5/7/2007 865 1163 High flow
1 High flow = Sample was not collected under base flow conditions (sample collected at flows greater that
25% flow exceedance frequency.
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APPENDIX B
PROJECTED FLOW EXCEEDANCE FREQUENCIES FOR
SULPHUR CREEK FLOW DURATION CURVE
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Appendix B
Projected Flow exceedance frequencies for Sulphur Creek Flow Duration Curve
WBID Segment OK410600010030_00
USGS Gage Reference 07332500
Flow Exceedance
Frequency (%) Flow (cfs)
Flow Exceedance
Frequency (%) Flow (cfs)
Flow Exceedance
Frequency (%) Flow (cfs)
0 1668.2 34 5.2 68 1.9
1 174.5 35 5.0 69 1.9
2 112.9 36 4.8 70 1.8
3 80.3 37 4.7 71 1.8
4 57.9 38 4.5 72 1.7
5 44.7 39 4.4 73 1.7
6 35.4 40 4.2 74 1.6
7 29.2 41 4.1 75 1.6
8 24.8 42 4.0 76 1.5
9 21.7 43 3.9 77 1.5
10 19.2 44 3.8 78 1.5
11 17.3 45 3.7 79 1.4
12 15.7 46 3.6 80 1.4
13 14.5 47 3.5 81 1.3
14 13.3 48 3.4 82 1.3
15 12.5 49 3.3 83 1.2
16 11.7 50 3.2 84 1.2
17 11.0 51 3.1 85 1.2
18 10.3 52 3.0 86 1.1
19 9.7 53 2.9 87 1.1
20 9.2 54 2.8 88 1.1
21 8.8 55 2.7 89 1.0
22 8.4 56 2.7 90 1.0
23 8.0 57 2.6 91 1.0
24 7.7 58 2.5 92 0.9
25 7.4 59 2.4 93 0.9
26 7.1 60 2.3 94 0.8
27 6.8 61 2.3 95 0.7
28 6.6 62 2.2 96 0.7
29 6.3 63 2.2 97 0.5
30 6.0 64 2.1 98 0.4
31 5.8 65 2.1 99 0.12
32 5.6 66 2.0 100 0
33 5.4 67 2.0
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Appendix B
General Method for Estimating Flow at WQM Stations
Flows duration curve will be developed using existing USGS measured flow where the
data exist from a gage on the stream segment of interest, or by estimating flow for stream
segments with no corresponding flow record. Flow data to support flow duration curves and
load duration curves will be derived for each Oklahoma stream segment in the following
priority:
i) In cases where a USGS flow gage occurs on, or within one-half mile upstream or
downstream of the Oklahoma stream segment.
a. If simultaneously collected flow data matching the water quality sample
collection date are available, these flow measurements will be used.
b. If flow measurements at the coincident gage are missing for some dates on
which water quality samples were collected, the gaps in the flow record will be
filled, or the record will be extended, by estimating flow based on measured
streamflows at a nearby gage. First, the most appropriate nearby stream gage is
identified. All flow data are first log-transformed to linearize the data because
flow data are highly skewed. Linear regressions are then developed between 1)
daily streamflow at the gage to be filled/extended, and 2) streamflow at all gages
within 95 miles that have at least 300 daily flow measurements on matching
dates. The station with the best flow relationship, as indicated by the highest r-squared
value, is selected as the index gage. R-squared indicates the fraction of
the variance in flow explained by the regression. The regression is then used to
estimate flow at the gage to be filled/extended from flow at the index station.
Flows will not be estimated based on regressions with r-squared values less than
0.25, even if that is the best regression. In some cases, it will be necessary to
fill/extend flow records from two or more index gages. The flow record will be
filled/extended to the extent possible based on the best index gage (highest r-squared
value), and remaining gaps will be filled from the next best index gage
(second highest r-squared value), and so forth.
c. Flow duration curves will be based on both measured flows only and on the
filled or extended flow time series calculated from other gages using regression.
d. On a stream impounded by dams to form reservoirs of sufficient size to impact
stream flow, only flows measured after the date of the most recent impoundment
will be used to develop the flow duration curve. This also applies to reservoirs
on major tributaries to the stream.
ii) In the case no coincident flow data are available for a stream segment, but flow
gage(s) are present upstream and/or downstream without a major reservoir between,
flows will be estimated for the stream segment from an upstream or downstream
gage using a watershed area ratio method derived by delineating subwatersheds, and
relying on the NRCS runoff curve numbers and antecedent rainfall condition.
Drainage subbasins will first be delineated for all impaired 303(d)-listed WQM
stations, along with all USGS flow stations located in the 8-digit HUCs with
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impaired streams. Parsons will then identify all the USGS gage stations upstream
and downstream of the subwatersheds with 303(d) listed WQM stations.
a. Watershed delineations are performed using ESRI Arc Hydro with a 30 m
resolution National Elevation Dataset (NED) digital elevation model, and
National Hydrography Dataset (NHD) streams. The area of each watershed will
be calculated following watershed delineation.
b. The watershed average curve number is calculated from soil properties and land
cover as described in the U.S. Department of Agriculture (USDA) Publication
TR-55: Urban Hydrology for Small Watersheds. The soil hydrologic group is
extracted from NRCS STATSGO soil data, and land use category from the 2001
National Land Cover Dataset (NLCD). Based on land use and the hydrologic
soil group, SCS curve numbers are estimated at the 30-meter resolution of the
NLCD grid as shown in Table 7. The average curve number is then calculated
from all the grid cells within the delineated watershed.
c. The average rainfall is calculated for each watershed from gridded average
annual precipitation datasets for the period 1971-2000 (Spatial Climate Analysis
Service, Oregon State University, http://www.ocs.oregonstate.edu/prism/,
created 20 Feb 2004).
Table B-1 Runoff Curve Numbers for Various Land Use Categories and Hydrologic Soil
Groups
NLCD Land Use Category
Curve number for hydrologic soil group
A B C D
0 in case of zero 100 100 100 100
11 Open Water 100 100 100 100
12 Perennial Ice/Snow 100 100 100 100
21 Developed, Open Space 39 61 74 80
22 Developed, Low Intensity 57 72 81 86
23 Developed, Medium Intensity 77 85 90 92
24 Developed, High Intensity 89 92 94 95
31 Barren Land (Rock/Sand/Clay) 77 86 91 94
32 Unconsolidated Shore 77 86 91 94
41 Deciduous Forest 37 48 57 63
42 Evergreen Forest 45 58 73 80
43 Mixed Forest 43 65 76 82
51 Dwarf Scrub 40 51 63 70
52 Shrub/Scrub 40 51 63 70
71 Grasslands/Herbaceous 40 51 63 70
72 Sedge/Herbaceous 40 51 63 70
73 Lichens 40 51 63 70
74 Moss 40 51 63 70
81 Pasture/Hay 35 56 70 77
82 Cultivated Crops 64 75 82 85
90-99 Wetlands 100 100 100 100
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d. The method used to project flow from a gaged location to an ungaged location was
adapted by combining aspects of two other flow projection methodologies
developed by Furness (Furness, 1959) and Wurbs (Wurbs, 2000).
Furness Method
The Furness method has been employed in Kansas by both the USGS and Kansas
Department of Health and Environment to estimate flow-duration curves. The method
typically uses maps, graphs, and computations to identify six unique factors of flow
duration for ungaged sites. These factors include:
the mean streamflow and percentage duration of mean streamflow;
the ratio of 1-percent-duration streamflow to mean streamflow ;
the ratio of 0.1-percent-duration streamflow to 1-percent-duration streamflow;
the ratio of 50-percentduration streamflow to mean streamflow;
the percentage duration of appreciable (0.10 ft /s) streamflow; and
average slope of the flow-duration curve.
Furness defined appreciable flow as 0.10 ft/s. This value of streamflow was
important because, for many years, this was the smallest non-zero streamflow value
reported in most Kansas streamflow records. The average slope of the duration curve is
a graphical approximation of the variability index, which is the standard deviation of the
logarithms of the streamflows (Furness, 1959, p. 202-204, figs. 147 and 148). On a
duration curve that fits the log-normal distribution exactly, the variability index is equal
to the ratio of the streamflow at the 15.87-percent-duration point to the streamflow at
the 50-percent-duration point. Because duration curves usually do not exactly fit the
log-normal distribution, the average-slope line is drawn through an arbitrary point, and
the slope is transferred to a position approximately defined by the previously estimated
points.
The method provides a means of both describing shape of the flow duration curve
and scaling the magnitude of the curve to another location, basically generating a new
flow duration curve with a very similar shape but different magnitude at the ungaged
location.
Wurbs Modified NRCS Method
As a part of the Texas water availability modeling (WAM) system developed by
Texas Natural Resources Conservation Commission (TNRCC), now known as the
Texas Commission on Environmental Quality (TCEQ), and partner agencies, various
contractors developed models of all Texas rivers. As a part of developing the model
code to be used, Dr. Ralph Wurbs of Texas A&M University researched methods to
distribute flows from gaged locations to ungaged locations. (Wurbs, 2006) His results
included the development of a modified Natural Resource Conservation Service
(NRCS) curve-number (CN) method for distributing flows from gaged locations to
ungaged locations.
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This modified NRCS method is based on the following relationship between
rainfall depth, P in inches, and runoff depth, Q in inches (NRCS, 1985; McCuen, 2005):
(P I ) S
(P I )
Q
a
2
a (1)
where:
Q = runoff depth (inches)
P = rainfall (inches)
S = potential maximum retention after runoff begins (inches)
Ia = initial abstraction (inches)
If P < 0.2, Q = 0. Initial abstraction has been found to be empirically related to S
by the equation
Ia = 0.2*S (2)
Thus, the runoff curve number equation can be rewritten:
P 0.8S
(P 0.2S)
Q
2
(3)
S is related to the curve number (CN) by:
10
CN
1000
S (4)
P and Q in inches must be multiplied by the watershed area to obtain volumes. The
potential maximum retention, S in inches, represents an upper limit on the amount of
water that can be abstracted by the watershed through surface storage, infiltration, and
other hydrologic abstractions. For convenience, S is expressed in terms of a curve
number CN, which is a dimensionless watershed parameter ranging from 0 to 100. A
CN of 100 represents a limiting condition of a perfectly impervious watershed with zero
retention and thus all the rainfall becoming runoff. A CN of zero conceptually
represents the other extreme with the watershed abstracting all rainfall with no runoff
regardless of the rainfall amount.
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First, S is calculated from the average curve number for the gaged watershed. Next,
the daily historic flows at the gage are converted to depth basis (as used in equations 1
and 3) by dividing by its drainage area, then converted to inches. Equation 3 is then
solved for daily precipitation depth of the gaged site, Pgaged. The daily precipitation
depth for the ungaged site is then calculated as the precipitation depth of the gaged site
multiplied by the ratio of the long-term average precipitation in the watersheds of the
ungaged and gaged sites:
gaged
ungaged
ungaged gaged M
M
P P (5)
where M is the mean annual precipitation of the watershed in inches. The daily
precipitation depth for the ungaged watershed, along with the average curve number of
the ungaged watershed, are then used to calculate the depth equivalent daily flow Q of
the ungaged site. Finally, the volumetric flow rate at the ungaged site is calculated by
multiplying by the area of the watershed of the ungaged site and converted to cubic feet.
In a subsequent study (Wurbs, 2006), Wurbs evaluated the predictive ability of
various flow distribution methods including:
Distribution of flows in proportion to drainage area;
Flow distribution equation with ratios for various watershed parameters;
Modified NRCS curve-number method;
Regression equations relating flows to watershed characteristics;
Use of recorded data at gaging stations to develop precipitation-runoff
relationships; and
Use of watershed (precipitation-runoff) computer models such as SWAT.
As a part of the analysis, the methods were used to predict flows at one gaged
station to another gage station so that fit statistics could be calculated to evaluate the
efficacy of each of the methods. Based upon similar analyses performed for many
gaged sites which reinforced the tests performed as part of the study, Wurbs observed
that temporal variations in flows are dramatic, ranging from zero flows to major floods.
Mean flows are reproduced reasonably well with the all flow distribution methods and
the NRCS CN method reproduces the mean closest. Accuracy in predicting mean flows
is much better than the accuracy of predicting the flow-frequency relationship.
Performance in reproducing flow-frequency relationships is better than for reproducing
flows for individual flows.
Wurbs concluded that the NRCS CN method, the drainage area ratio method, and
drainage area – CN – mean annual precipitation depth (MP) ratio methods all yield
similar levels of accuracy. If the CN and MP are the same for the gaged and ungaged
watersheds, the three alternative methods yield identical results. Drainage area is the
most important watershed parameter. However, the NRCS method adaptation is
preferable in those situations in which differences in CN (land use and soil type) and
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long-term MP are significantly different between the gaged and ungaged watersheds.
The CN and MP are usually similar but not identical.
Generalized Flow Projection Methodology
In the first several versions of the TMDL toolbox, all flows at ungaged sites that
required projection from a gaged site were performed with the Modified NRCS CN
method. This led a number of problems with flow projections in the early versions. As
described previously, the NRCS method, in common with all others, reproduces the
mean or central tendency best but the accuracy of the fit degrades towards the extremes
of the frequency spectrum. Part of the degradation in accuracy is due to the quite non-linear
nature of the NRCS equations. On the low flow end of the frequency spectrum,
Equation 2 above constitutes a low flow limit below which the NRCS equations are not
applicable at all. Given the flashy nature of most streams in locations for which the
toolbox was developed, high and low flows are relatively more common and spurious
results from the limits of the equations abounded.
In an effort to increase the flow prediction efficacy and remedy the failure of the
NRCS CN method at the extremes of the flow spectrum, we developed what is
effectively a hybrid of the NRCS CN method and the Furness method. Noting the facts
that all tested projection methods, and particularly the NRCS CN method, perform best
near the central tendency or mean and that none of the methods predict the entire flow
frequency spectrum well, we decided to adopt an assumption that is implicit in the
Furness method. The Furness method implicitly assumes that the shape of the flow
frequency curve at an upstream site is related to and similar to the shape of the flow
frequency curve at site downstream. As described previously, the Furness method
employs several relationships derived between the mean flows and flows at differing
frequencies to replicate the shape of the flow frequency curve at the projected site,
while utilizing other regressed relationships to scale the magnitude of the curve. Since,
as part of the toolbox calculations, the entire flow frequency curve at a 1% interval is
calculated for every USGS gage utilizing very long periods of record, we decided to use
this vector in association with the mean flow to project the flow frequency curve.
In the ideal situation flows are projected from an ungaged location from a
downstream gaged location. The toolbox also has the capability to project flows from
and upstream gaged location if there is no useable downstream gage.
iii) In the rare case where no coincident flow data are available for a WQM station and no
gages are present upstream or downstream, flows will be estimated for the WQM
station from a gage on an adjacent watershed of similar size and properties, via the same
procedure described above for upstream or downstream gages.
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APPENDIX C
STATE OF OKLAHOMA ANTIDEGRADATION POLICY
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Appendix C
State of Oklahoma Antidegradation Policy
785:45-3-1. Purpose; Antidegradation policy statement
(a) Waters of the state constitute a valuable resource and shall be protected, maintained
and improved for the benefit of all the citizens.
(b) It is the policy of the State of Oklahoma to protect all waters of the state from
degradation of water quality, as provided in OAC 785:45-3-2 and Subchapter 13 of
OAC 785:46.
785:45-3-2. Applications of antidegradation policy
(a) Application to outstanding resource waters (ORW). Certain waters of the state
constitute an outstanding resource or have exceptional recreational and/or ecological
significance. These waters include streams designated "Scenic River" or "ORW" in
Appendix A of this Chapter, and waters of the State located within watersheds of
Scenic Rivers. Additionally, these may include waters located within National and
State parks, forests, wilderness areas, wildlife management areas, and wildlife
refuges, and waters which contain species listed pursuant to the federal Endangered
Species Act as described in 785:45-5-25(c)(2)(A) and 785:46-13-6(c). No degradation
of water quality shall be allowed in these waters.
(b) Application to high quality waters (HQW). It is recognized that certain waters of the
state possess existing water quality which exceeds those levels necessary to support
propagation of fishes, shellfishes, wildlife, and recreation in and on the water. These
high quality waters shall be maintained and protected.
(c) Application to beneficial uses. No water quality degradation which will interfere with
the attainment or maintenance of an existing or designated beneficial use shall be
allowed.
(d) Application to improved waters. As the quality of any waters of the state improve, no
degradation of such improved waters shall be allowed.
785:46-13-1. Applicability and scope
(a) The rules in this Subchapter provide a framework for implementing the
antidegradation policy stated in OAC 785:45-3-2 for all waters of the state. This
policy and framework includes three tiers, or levels, of protection.
(b) The three tiers of protection are as follows:
(1) Tier 1. Attainment or maintenance of an existing or designated beneficial use.
(2) Tier 2. Maintenance or protection of High Quality Waters and Sensitive Public
and Private Water Supply waters.
(3) Tier 3. No degradation of water quality allowed in Outstanding Resource
Waters.
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(c) In addition to the three tiers of protection, this Subchapter provides rules to implement
the protection of waters in areas listed in Appendix B of OAC 785:45. Although
Appendix B areas are not mentioned in OAC 785:45-3-2, the framework for
protection of Appendix B areas is similar to the implementation framework for the
antidegradation policy.
(d) In circumstances where more than one beneficial use limitation exists for a
waterbody, the most protective limitation shall apply. For example, all antidegradation
policy implementation rules applicable to Tier 1 waterbodies shall be applicable also
to Tier 2 and Tier 3 waterbodies or areas, and implementation rules applicable to Tier
2 waterbodies shall be applicable also to Tier 3 waterbodies.
(e) Publicly owned treatment works may use design flow, mass loadings or concentration,
as appropriate, to calculate compliance with the increased loading requirements of this
section if those flows, loadings or concentrations were approved by the Oklahoma
Department of Environmental Quality as a portion of Oklahoma's Water Quality
Management Plan prior to the application of the ORW, HQW or SWS limitation.
785:46-13-2. Definitions
The following words and terms, when used in this Subchapter, shall have the following
meaning, unless the context clearly indicates otherwise:
"Specified pollutants" means
(A) Oxygen demanding substances, measured as Carbonaceous Biochemical Oxygen
Demand (CBOD) and/or Biochemical Oxygen Demand (BOD);
(B) Ammonia Nitrogen and/or Total Organic Nitrogen;
(C) Phosphorus;
(D) Total Suspended Solids (TSS); and
(E) Such other substances as may be determined by the Oklahoma Water Resources
Board or the permitting authority.
785:46-13-3. Tier 1 protection; attainment or maintenance of an existing or designated
beneficial use
(a) General.
(1) Beneficial uses which are existing or designated shall be maintained and
protected.
(2) The process of issuing permits for discharges to waters of the state is one of
several means employed by governmental agencies and affected persons which
are designed to attain or maintain beneficial uses which have been designated
for those waters. For example, Subchapters 3, 5, 7, 9 and 11 of this Chapter are
rules for the permitting process. As such, the latter Subchapters not only
implement numerical and narrative criteria, but also implement Tier 1 of the
antidegradation policy.
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(b) Thermal pollution. Thermal pollution shall be prohibited in all waters of the state.
Temperatures greater than 52 degrees Centigrade shall constitute thermal pollution
and shall be prohibited in all waters of the state.
(c) Prohibition against degradation of improved waters. As the quality of any waters of
the state improves, no degradation of such improved waters shall be allowed.
785:46-13-4. Tier 2 protection; maintenance and protection of High Quality Waters and
Sensitive Water Supplies
(a) General rules for High Quality Waters. New point source discharges of any pollutant
after June 11, 1989, and increased load or concentration of any specified pollutant
from any point source discharge existing as of June 11, 1989, shall be prohibited in
any waterbody or watershed designated in Appendix A of OAC 785:45 with the
limitation "HQW". Any discharge of any pollutant to a waterbody designated "HQW"
which would, if it occurred, lower existing water quality shall be prohibited. Provided
however, new point source discharges or increased load or concentration of any
specified pollutant from a discharge existing as of June 11, 1989, may be approved by
the permitting authority in circumstances where the discharger demonstrates to the
satisfaction of the permitting authority that such new discharge or increased load or
concentration would result in maintaining or improving the level of water quality
which exceeds that necessary to support recreation and propagation of fishes,
shellfishes, and wildlife in the receiving water.
(b) General rules for Sensitive Public and Private Water Supplies. New point source
discharges of any pollutant after June 11, 1989, and increased load of any specified
pollutant from any point source discharge existing as of June 11, 1989, shall be
prohibited in any waterbody or watershed designated in Appendix A of OAC 785:45
with the limitation "SWS". Any discharge of any pollutant to a waterbody designated
"SWS" which would, if it occurred, lower existing water quality shall be prohibited.
Provided however, new point source discharges or increased load of any specified
pollutant from a discharge existing as of June 11, 1989, may be approved by the
permitting authority in circumstances where the discharger demonstrates to the
satisfaction of the permitting authority that such new discharge or increased load will
result in maintaining or improving the water quality in both the direct receiving water,
if designated SWS, and any downstream waterbodies designated SWS.
(c) Stormwater discharges. Regardless of subsections (a) and (b) of this Section, point
source discharges of stormwater to waterbodies and watersheds designated "HQW"
and "SWS" may be approved by the permitting authority.
(d) Nonpoint source discharges or runoff. Best management practices for control of
nonpoint source discharges or runoff should be implemented in watersheds of
waterbodies designated "HQW" or "SWS" in Appendix A of OAC 785:45.
785:46-13-5. Tier 3 protection; prohibition against degradation of water quality in
outstanding resource waters
(a) General. New point source discharges of any pollutant after June 11, 1989, and
increased load of any pollutant from any point source discharge existing as of June 11,
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1989, shall be prohibited in any waterbody or watershed designated in Appendix A of
OAC 785:45 with the limitation "ORW" and/or "Scenic River", and in any waterbody
located within the watershed of any waterbody designated with the limitation "Scenic
River". Any discharge of any pollutant to a waterbody designated "ORW" or "Scenic
River" which would, if it occurred, lower existing water quality shall be prohibited.
(b) Stormwater discharges. Regardless of 785:46-13-5(a), point source discharges of
stormwater from temporary construction activities to waterbodies and watersheds
designated "ORW" and/or "Scenic River" may be permitted by the permitting
authority. Regardless of 785:46-13-5(a), discharges of stormwater to waterbodies and
watersheds designated "ORW" and/or "Scenic River" from point sources existing as
of June 25, 1992, whether or not such stormwater discharges were permitted as point
sources prior to June 25, 1992, may be permitted by the permitting authority;
provided, however, increased load of any pollutant from such stormwater discharge
shall be prohibited.
(c) Nonpoint source discharges or runoff. Best management practices for control of
nonpoint source discharges or runoff should be implemented in watersheds of
waterbodies designated "ORW" in Appendix A of OAC 785:45, provided, however,
that development of conservation plans shall be required in sub-watersheds where
discharges or runoff from nonpoint sources are identified as causing or significantly
contributing to degradation in a waterbody designated "ORW".
(d) LMFO's. No licensed managed feeding operation (LMFO) established after June 10,
1998 which applies for a new or expanding license from the State Department of
Agriculture after March 9, 1998 shall be located...[w]ithin three (3) miles of any
designated scenic river area as specified by the Scenic Rivers Act in 82 O.S. Section
1451 and following, or [w]ithin one (1) mile of a waterbody [2:9-210.3(D)]
designated in Appendix A of OAC 785:45 as "ORW".
785:46-13-6. Protection for Appendix B areas
(a) General. Appendix B of OAC 785:45 identifies areas in Oklahoma with waters of
recreational and/or ecological significance. These areas are divided into Table 1,
which includes national and state parks, national forests, wildlife areas, wildlife
management areas and wildlife refuges; and Table 2, which includes areas which
contain threatened or endangered species listed as such by the federal government
pursuant to the federal Endangered Species Act as amended.
(b) Protection for Table 1 areas. New discharges of pollutants after June 11, 1989, or
increased loading of pollutants from discharges existing as of June 11, 1989, to waters
within the boundaries of areas listed in Table 1 of Appendix B of OAC 785:45 may be
approved by the permitting authority under such conditions as ensure that the
recreational and ecological significance of these waters will be maintained.
(c) Protection for Table 2 areas. Discharges or other activities associated with those
waters within the boundaries listed in Table 2 of Appendix B of OAC 785:45 may be
restricted through agreements between appropriate regulatory agencies and the United
States Fish and Wildlife Service. Discharges or other activities in such areas shall not
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substantially disrupt the threatened or endangered species inhabiting the receiving
water.
(d) Nonpoint source discharges or runoff. Best management practices for control of
nonpoint source discharges or runoff should be implemented in watersheds located
within areas listed in Appendix B of OAC 785:45.

FINAL
TURBIDITY TOTAL MAXIMUM DAILY LOADS FOR SULPHUR
CREEK, OKLAHOMA (OK410600010030_00)
Prepared for:
OKLAHOMA DEPARTMENT OF ENVIRONMENTAL QUALITY
Prepared by:
AUGUST 2010
FINAL
TURBIDITY TOTAL MAXIMUM DAILY LOADS FOR SULPHUR
CREEK, OKLAHOMA (OK410600010030_00)
OKWBID
OK410600010030_00
Prepared for:
OKLAHOMA DEPARTMENT OF ENVIRONMENTAL QUALITY
Prepared by:
8000 Centre Park Drive, Suite 200
Austin, TX 78754
AUGUST 2010
Oklahoma Department of Environmental Quality: FY07/08 106 Carryover Grant (I-006400-08)
Funding for the development of this TMDL Report was provided through a federal Clean Water Act grant to
the Oklahoma Department of Environmental Quality from the U.S. Environmental Protection Agency.
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TABLE OF CONTENTS
EXECUTIVE SUMMARY ................................................................................................. ES-1
SECTION 1 INTRODUCTION ............................................................................................. 1-1
1.1 TMDL Program Background ..................................................................................... 1-1
1.2 Watershed Description ............................................................................................... 1-4
1.3 Stream Flow Data ....................................................................................................... 1-7
SECTION 2 PROBLEM IDENTIFICATION AND WATER QUALITY TARGET ...... 2-1
2.1 Oklahoma Water Quality Standards ........................................................................... 2-1
2.2 Problem Identification ................................................................................................ 2-3
2.3 Water Quality Target .................................................................................................. 2-4
SECTION 3 POLLUTANT SOURCE ASSESSMENT ....................................................... 3-1
3.1 NPDES-Permitted Facilities ....................................................................................... 3-1
3.1.1 Continuous Point Source Discharges ............................................................. 3-2
3.1.2 Concentrated Animal Feeding Operations ..................................................... 3-2
3.1.3 Stormwater Permits for MA4 and Construction Activities ............................ 3-2
3.1.4 Section 404 permits ........................................................................................ 3-2
3.2 Nonpoint Sources ....................................................................................................... 3-3
SECTION 4 TECHNICAL APPROACH AND METHODS .............................................. 4-1
4.1 Determining a Surrogate Target ................................................................................. 4-1
4.2 Using Load Duration Curves to Develop TMDLs ..................................................... 4-4
4.3 Development of Flow Duration Curves ..................................................................... 4-4
4.4 Development of TMDLs Using Load Duration Curves ............................................. 4-6
SECTION 5 TMDL CALCULATIONS ................................................................................ 5-1
5.1 Estimated Loading and Critical Conditions ............................................................... 5-1
5.2 Wasteload Allocation ................................................................................................. 5-2
5.2.1 Section 404 permits ........................................................................................ 5-2
5.3 Load Allocation .......................................................................................................... 5-3
5.4 Seasonal Variability .................................................................................................... 5-3
5.5 Margin of Safety ......................................................................................................... 5-3
5.6 TMDL Calculations .................................................................................................... 5-3
5.7 Reasonable Assurances .............................................................................................. 5-4
SECTION 6 PUBLIC PARTICIPATION ............................................................................ 6-1
SECTION 7 REFERENCES .................................................................................................. 7-1
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APPENDICES
Appendix A Ambient Water Quality Data 1991 - 2007
Appendix B Projected Flow exceedance frequencies for Sulphur Creek Flow Duration Curve
Appendix C State of Oklahoma Antidegradation Policy
Appendix D Response to Public Comments
LIST OF FIGURES
Figure 1-1 Watersheds Not Supporting Fish and Wildlife Propagation Use
within the Study Area ........................................................................................... 1-3
Figure 1-2 Land Use Map by Watershed ............................................................................... 1-6
Figure 3-1 Locations of Permitted Facilities in the Study Area ............................................. 3-4
Figure 4-1 Linear Regression for TSS-Turbidity under Base-flow Conditions for Sulphur
Creek (OK 410600010030_00) ............................................................................ 4-3
Figure 4-2 Flow Duration Curve for Sulphur Creek (OK410600010030_00) ....................... 4-6
Figure 5-1 Load Duration Curve for Total Suspended Solids in Sulphur Creek
(OK410600010030_00) ........................................................................................ 5-2
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LIST OF TABLES
Table ES-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List ................................................................................. ES-2
Table ES-2 Summary of Turbidity Samples Collected During Base Flow Conditions 1998 -
2007 ................................................................................................................... ES-2
Table ES-3 Summary of TSS Samples Collected During Base Flow Conditions 1998 -
2007 ................................................................................................................... ES-3
Table ES-4 Turbidity TMDLs based on Total Suspended Solids Calculations for Sulphur
Creek (OK410600010030_00) .......................................................................... ES-6
Table 1-1 Water Quality Monitoring Stations used for 2008 303(d) Listing Decision ........ 1-2
Table 1-2 County Population and Density ............................................................................ 1-4
Table 1-3 Average Annual Precipitation by Watershed ....................................................... 1-4
Table 1-4 Land Use Summaries by Watershed ..................................................................... 1-5
Table 2-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List .................................................................................... 2-2
Table 2-2 Summary of All Turbidity Samples 1991 - 2007 ................................................. 2-3
Table 2-3 Summary of Turbidity Samples Collected During Base Flow Conditions 1992 -
2007 ...................................................................................................................... 2-3
Table 2-4 Summary of All TSS Samples 1991 - 2007 ......................................................... 2-4
Table 2-5 Summary of TSS Samples Collected During Base Flow Conditions 1992 -
2007 ...................................................................................................................... 2-4
Table 3-1 Stormwater Permits for Construction Activities .................................................. 3-2
Table 5-1 Turbidity TMDL based on Total Suspended Solids Calculations for Sulphur Creek
(OK410600010030_00) ........................................................................................ 5-4
Table 5-2 Partial List of Oklahoma Water Quality Management Agencies ......................... 5-5
Sulphur Creek TMDL Acronyms and Abbreviations
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April 2010
ACRONYMS AND ABBREVIATIONS
BMP best management practice
CAFO Concentrated Animal Feeding Operation
CFR Code of Federal Regulations
cfs Cubic feet per second
CPP Continuing planning process
CWA Clean Water Act
DMR Discharge monitoring report
IQR interquartile range
LA Load allocation
LDC Load duration curve
LOC line of organic correlation
mg Million gallons
mgd Million gallons per day
mg/L microgram per liter
MOS Margin of safety
MS4 Municipal separate storm sewer system
NPDES National Pollutant Discharge Elimination System
NRCS National Resources Conservation Service
NTU nephelometric turbidity unit
OLS ordinary least square regression
O.S. Oklahoma statutes
ODAFF Oklahoma Department of Agriculture, Food and Forestry
ODEQ Oklahoma Department of Environmental Quality
OPDES Oklahoma Pollutant Discharge Elimination System
OWRB Oklahoma Water Resources Board
PRG Percent reduction goal
TMDL Total maximum daily load
TSS Total suspended solids
USDA U.S. Department of Agriculture
USEPA U.S. Environmental Protection Agency
USGS U.S. Geological Survey
WLA Wasteload allocation
WQM Water quality monitoring
WQS Water quality standard
WWTP Wastewater treatment plant
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EXECUTIVE SUMMARY
This report documents the data and assessment used to establish a TMDL for Sulphur
Creek, a tributary of the Blue River. The 2008 Integrated Water Quality Assessment Report
(Oklahoma Department of Environmental Quality [ODEQ] 2008) identified Sulphur Creek as
impaired for turbidity. Data assessment and TMDL calculations are conducted in accordance
with requirements of Section 303(d) of the CWA, Water Quality Planning and Management
Regulations (40 CFR Part 130), USEPA guidance, and ODEQ guidance and procedures.
ODEQ is required to submit all TMDLs to USEPA for review and approval. Once the USEPA
approves a TMDL, the waterbody may be moved to Category 4a of a state’s Integrated Water
Quality Monitoring and Assessment Report, where it remains until compliance with water
quality standards (WQS) is achieved (USEPA 2003).
The purpose of this TMDL report is to establish pollutant load allocations for turbidity in
impaired waterbodies, which is the first step toward restoring water quality. TMDLs determine
the pollutant loading a waterbody can assimilate without exceeding the WQS for that pollutant.
TMDLs also establish the pollutant load allocation necessary to meet the WQS established for a
waterbody based on the relationship between pollutant sources and in-stream water quality
conditions. A TMDL consists of a wasteload allocation (WLA), load allocation (LA), and a
margin of safety (MOS). The WLA is the fraction of the total pollutant load apportioned to
point sources, and includes stormwater discharges regulated under the National Pollutant
Discharge Elimination System (NPDES) as point sources. The LA is the fraction of the total
pollutant load apportioned to nonpoint sources. The MOS is a percentage of the TMDL set
aside to account for the lack of knowledge associated with natural process in aquatic systems,
model assumptions, and data limitations.
This report does not stipulate specific control actions (regulatory controls) or management
measures (voluntary best management practices) necessary to reduce turbidity loadings within
each watershed. Watershed-specific control actions and management measures will be
identified, selected, and implemented under a separate process involving stakeholders who live
and work in the watershed; tribes; and local, state, and federal government agencies.
E.1 Problem Identification and Water Quality Target
The TMDL in this report address fish and wildlife propagation for the subcategory warm water
aquatic community. Table ES-1, an excerpt from Appendix B of the 2008 Integrated Report
(ODEQ 2008), summarizes the warm water aquatic community use attainment status and the
scheduled date for TMDL development established by ODEQ for the impaired waterbody of
the Study Area.
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Table ES-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List
Waterbody ID Waterbody Name
Stream Miles
Category
TMDL Date
Priority
Warm Water
Aquatic
Community
OK410600010030_00 Sulphur Creek 14.6 5a 2013 2 N
N = Not Supporting;
5a = TMDL is underway or will be scheduled
Source: 2008 Integrated Report, ODEQ 2008
The data in Table ES-2 were used to support the decision to place Sulphur Creek on the
ODEQ 2008 303(d) list (ODEQ 2008 for nonsupport of the Fish and Wildlife Propagation use
based on turbidity levels observed in the waterbody. Turbidity is a measure of water clarity
and is caused by suspended particles in the water column. Because turbidity cannot be
expressed as a mass load, total suspended solids (TSS) are used as a surrogate in this TMDL.
Therefore, both turbidity and TSS data are presented to support TMDL development.
The numeric criteria for turbidity to maintain and protect the use of “Fish and Wildlife
Propagation” from Title 785:45-5-12 (f) (7) is as follows:
(A) Turbidity from other than natural sources shall be restricted to not exceed the following
numerical limits:
1. Cool Water Aquatic Community/Trout Fisheries: 10 NTUs;
2. Lakes: 25 NTU; and
3. Other surface waters: 50 NTUs.
(B) In waters where background turbidity exceeds these values, turbidity from point sources
will be restricted to not exceed ambient levels.
(C) Numerical criteria listed in (A) of this paragraph apply only to seasonal base flow
conditions.
(D) Elevated turbidity levels may be expected during, and for several days after, a runoff event.
Table ES-2 Summary of Turbidity Samples Collected During Base Flow Conditions
1998 - 2007
WQM Station
Number of Turbidity
Samples Collected
During Base Flow
Conditions
Number of
Samples
Exceeding 50 NTU
during Base Flow
Conditions
Percentage of
Samples
Exceeding
Criterion
during Base
Flow
Conditions
Average Turbidity
(NTU) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 19 2 11% 34
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Table ES-3 presents a subset of total suspended solids data for samples collected during
base flow conditions. Water quality data for turbidity and TSS are provided in Appendix A.
Table ES-3 Summary of TSS Samples Collected During Base Flow Conditions
1998 -2007
WQM Station
Number of TSS
Samples Collected
During Base Flow
Conditions
Average TSS
(mg/L) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 18 19
The Code of Federal Regulations (40 CFR §130.7(c)(1)) states that, “TMDLs shall be
established at levels necessary to attain and maintain the applicable narrative and numerical
water quality standards.” An individual water quality target established for turbidity must
demonstrate compliance with the numeric criteria prescribed in the Oklahoma WQS
(OWRB 2008). According to the Oklahoma WQS [785:45-5-12(f)(7)], the turbidity criterion
for streams with warm water aquatic community (WWAC) beneficial use is 50 NTUs
(OWRB 2008). The turbidity of 50 NTUs applies only to seasonal base flow conditions.
Turbidity levels are expected to be elevated during, and for several days after, a storm event.
TMDLs for turbidity in streams designated as warm water aquatic community must take
into account that no more than 10 percent of the samples may exceed the numeric criterion of
50 NTU. However, as described above, because turbidity cannot be expressed as a mass load,
TSS is used as a surrogate in this TMDL. Since there is no numeric criterion in the Oklahoma
WQS for TSS, a specific method must be developed to convert the turbidity criterion to TSS
based on a relationship between turbidity and TSS. The method for deriving the relationship
between turbidity and TSS, and for calculating a water body specific water quality target using
TSS, is summarized in Section 4 of this report.
E.2 Pollutant Source Assessment
A pollutant source assessment characterizes known and suspected sources of pollutant
loading to impaired waterbodies. Sources within a watershed are categorized and quantified to
the extent that information is available. Turbidity may originate from NPDES-permitted
facilities, fields, construction sites, quarries, stormwater runoff and eroding stream banks. The
2008 Integrated Water Quality Assessment Report (ODEQ 2008) listed potential sources of
turbidity in Sulphur Creek (OK410600010030_00) as grazing in riparian corridors of streams
and creeks, highway/road/bridge runoff (non-construction related), non-irrigated crop
production, rangeland grazing, and other unknown sources.
There are no NPDES-permitted facilities and no municipal separate storm sewer systems
or CAFOs in the Study Area.
The relative homogeneous land use/land cover categories within the Study Area are
associated with agricultural and range management activities. This suggests that various
nonpoint sources of TSS include sediments originating from grazing in riparian corridors of
streams and creeks, highway/road/bridge runoff (non-construction related), non-irrigated crop
production, rangeland grazing and other sources of sediment loading (ODEQ 2008). Elevated
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turbidity measurements can be caused by stream bank erosion processes, stormwater runoff
events and other channel disturbances. However, there is insufficient data available to quantify
contributions of TSS from these processes. TSS or sediment loading can also occur under non-runoff
conditions as a result of anthropogenic activities in riparian corridors which cause
erosive conditions. Sediment loading of streams can also originate from natural erosion
processes, including the weathering of soil, rocks, and uncultivated land; geological abrasion;
and other natural phenomena. Given the lack of data to establish the background conditions for
TSS/turbidity, separating background loading from nonpoint sources is not feasible in this
TMDL development.
E.3 Using Load Duration Curves to Develop TMDLs
Turbidity is a commonly measured indicator of the suspended solids load in streams.
However, turbidity is an optical property of water, and measures scattering of light by
suspended solids and colloidal matter. To develop TMDLs, a gravimetric (mass-based)
measure of solids loading is required to express loads. There is often a strong relationship
between the total suspended solids concentration and turbidity. Therefore, the TSS load, which
is expressed as mass per time, is used as a surrogate for turbidity and represents the maximum
one-day load the stream can assimilate while still attaining the WQS.
To determine the relationship between turbidity and TSS, a linear regression between TSS
and turbidity was developed using data collected from 1998 to 2007 at one station within the
Study Area. Prior to developing the regression the following steps were taken to refine the
dataset:
Assign values to censored data (i.e., measured concentrations lower than the analytical
quantitation limit and, thus, reported as less than the quantitation limit). For example,
using 9.99 to replace all samples reported as “<10”;
Remove data collected under high flow conditions exceeding the base-flow criterion.
This means that measurements corresponding to flow exceedance frequencies lower
than 25% were not used in the regression;
Check rainfall data on the day when samples were collected and on the previous two
days. If there was a significant rainfall event (>= 1.0 inch) in any of these days, the
sample will be excluded from regression analysis with one exception. If the significant
rainfall happened on the sampling day and the turbidity reading was less than 25 NTUs
(half of turbidity standard for streams), the sample will not be excluded from analysis
because most likely the rainfall occurred after the sample was taken; andLog-transform
both turbidity and TSS data to minimize effects of their non-linear data
distributions.
The TMDL calculations presented in this report are derived from load duration curves
(LDC). LDCs facilitate rapid development of TMDLs, and as a TMDL development tool, are
effective at identifying whether impairments are associated with point or nonpoint sources.
The basic steps to generating an LDC involve:
obtaining daily flow data for the site of interest from the USGS (or project flow using
Oklahoma TMDL Toolbox if station is ungaged);
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sorting the flow data and calculating flow exceedance frequencies for the time period
and season of interest;
obtaining available turbidity and TSS water quality data;
matching the water quality observations with the flow data from the same date;
displaying a curve on a plot that represents the allowable load multiplying the actual or
estimated flow by the WQtarget for TSS;
multiplying the flow by the water quality parameter concentration to calculate daily
loads (for sampling events with both TSS and turbidity data, the measured TSS value
is used; if only turbidity was measured, the value was converted to TSS using the
regression equation in Figure 4-1); then
plotting the flow exceedance frequencies and daily load observations in a load duration
plot.
The culmination of these steps is expressed in the following formula, which is displayed on
the LDC as the TMDL curve:
TMDL (lb/day) = WQtarget * flow (cfs) * unit conversion factor
where: WQtarget = 31 mg/L (TSS)
unit conversion factor = 5.39377 L*s*lb /(ft3*day*mg)
The flow exceedance frequency (x-value of each point) is obtained by looking up the
historical exceedance frequency of the measured or estimated flow; in other words, the percent
of historical observations that equal or exceed the measured or estimated flow. Historical
observations of TSS and/or turbidity concentrations are paired with flow data and are plotted on
the LDC. The TSS load (or the y-value of each point) is calculated by multiplying the TSS
concentration (measured or converted from turbidity) (mg/L) by the instantaneous flow (cfs) at
the same site and time, with appropriate volumetric and time unit conversions. TSS loads
representing exceedance of water quality criteria fall above the water quality criterion line.
E.4 TMDL Calculations
The objective of a TMDL is to estimate allowable pollutant loads and to allocate these
loads to the known pollutant sources in the watershed so appropriate control measures can be
implemented and the WQS achieved. A TMDL is expressed as the sum of three elements as
described in the following mathematical equation:
TMDL = Σ WLA + Σ LA + MOS
The WLA is the portion of the TMDL allocated to existing and future point sources. The
LA is the portion of the TMDL allocated to nonpoint sources, including natural background
sources. The MOS is intended to ensure that WQS will be met. Thus, the allowable pollutant
load that can be allocated to point and nonpoint sources can then be defined as the TMDL
minus the MOS.
The overall Percent Reduction Goal (PRG) is calculated as the reduction in load required
so no more than 10 percent of the samples collected under base-flow conditions would exceed
TMDL targets for TSS. The PRG for Sulphur Creek is calculated to be 11.7 percent.
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The maximum assimilative capacity of a stream depends on the flow conditions of the
stream. The higher the flow is, the more wasteload the stream can handle without violating
water quality standards. Thus, the TMDL, WLA, LA, and MOS will vary with flow condition,
and are calculated at every 5th flow interval percentile (Table ES-4).
Table ES-4 Turbidity TMDLs based on Total Suspended Solids Calculations for
Sulphur Creek (OK410600010030_00)
Percentile
Flow
(cfs)
TMDL
(lb/day)
WLA (lb/day) LA
(lb/day)
MOS
WWTP MS4 Growth (lb/day)
0 1,668 NA 0 0 NA NA NA
5 45 NA 0 0 NA NA NA
10 19 NA 0 0 NA NA NA
15 12 NA 0 0 NA NA NA
20 9.2 NA 0 0 NA NA NA
25 7.4 1,253 0 0 12.5 1,115 125
30 6 1,016 0 0 10.2 904 102
35 5 847 0 0 8.5 754 85
40 4.2 711 0 0 7.1 633 71
45 3.7 627 0 0 6.3 558 63
50 3.2 542 0 0 5.4 482 54
55 2.7 457 0 0 4.6 407 46
60 2.3 390 0 0 3.9 347 39
65 2.1 356 0 0 3.6 317 36
70 1.8 305 0 0 3.0 271 30
75 1.6 271 0 0 2.7 241 27
80 1.4 237 0 0 2.4 211 24
85 1.2 203 0 0 2.0 181 20
90 1 169 0 0 1.7 151 17
95 0.7 119 0 0 1.2 106 12
100 0 0 0 0 0 0 0
E.5 Reasonable Assurance
ODEQ will collaborate with a host of other state agencies and local governments working
within the boundaries of state and local regulations to target available funding and technical
assistance to support implementation of pollution controls and management measures. Various
water quality management programs and funding sources provide a reasonable assurance that
the pollutant reductions as required by this TMDL can be achieved and water quality can be
restored to maintain designated uses. ODEQ’s Continuing Planning Process (CPP), required by
the CWA §303(e)(3) and 40 CFR 130.5, summarizes Oklahoma’s commitments and programs
aimed at restoring and protecting water quality throughout the state (ODEQ 2006). The CPP
can be viewed from ODEQ’s website at 2006 Continuing Planning Process. Table 5-2 provides
a partial list of the state partner agencies ODEQ will collaborate with to address point and
nonpoint source reduction goals established by TMDLs.
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SECTION 1
INTRODUCTION
1.1 TMDL Program Background
Section 303(d) of the Clean Water Act (CWA) and U.S. Environmental Protection Agency
(USEPA) Water Quality Planning and Management Regulations (40 Code of Federal
Regulations [CFR] Part 130) require states to develop total maximum daily loads (TMDL) for
waterbodies not meeting designated uses where technology-based controls are in place.
TMDLs establish the allowable loadings of pollutants or other quantifiable parameters for a
waterbody based on the relationship between pollution sources and in-stream water quality
conditions, so states can implement water quality-based controls to reduce pollution from point
and nonpoint sources and restore and maintain water quality (USEPA 1991).
This report documents the data and assessment used to establish a turbidity TMDL for
Sulphur Creek, a tributary of the Blue River. The 2008 Integrated Water Quality Assessment
Report (Oklahoma Department of Environmental Quality [ODEQ] 2008) identified Sulphur
Creek as impaired for turbidity. Data assessment and TMDL calculations are conducted in
accordance with requirements of Section 303(d) of the CWA, Water Quality Planning and
Management Regulations (40 CFR Part 130), USEPA guidance, and ODEQ guidance and
procedures. ODEQ is required to submit all TMDLs to USEPA for review and approval. Once
the USEPA approves a TMDL, the waterbody may be moved to Category 4a of a state’s
Integrated Water Quality Monitoring and Assessment Report, where it remains until
compliance with water quality standards (WQS) is achieved (USEPA 2003).
The purpose of this TMDL report is to establish pollutant load allocations for turbidity in
impaired waterbodies, which is the first step toward restoring water quality. TMDLs determine
the pollutant loading a waterbody can assimilate without exceeding the WQS for that pollutant.
TMDLs also establish the pollutant load allocation necessary to meet the WQS established for a
waterbody based on the relationship between pollutant sources and in-stream water quality
conditions. A TMDL consists of a wasteload allocation (WLA), load allocation (LA), and a
margin of safety (MOS). The WLA is the fraction of the total pollutant load apportioned to
point sources, and includes stormwater discharges regulated under the National Pollutant
Discharge Elimination System (NPDES) as point sources. The LA is the fraction of the total
pollutant load apportioned to nonpoint sources. The MOS is a percentage of the TMDL set
aside to account for the lack of knowledge associated with natural process in aquatic systems,
model assumptions, and data limitations.
This report does not stipulate specific control actions (regulatory controls) or management
measures (voluntary best management practices) necessary to reduce turbidity loadings within
each watershed. Watershed-specific control actions and management measures will be
identified, selected, and implemented under a separate process involving stakeholders who live
and work in the watershed; tribe;, and local, state, and federal government agencies.
This TMDL report focuses on waterbodies that ODEQ placed in Category 5 [303(d) list] of
the Water Quality in Oklahoma, 2008 Integrated Report (2008 Integrated Report) for the
beneficial use category Fish and Wildlife Propagation for Sulphur Creek
OK410600010030_00. Figure 1-1 is a location map showing the impaired segment of this
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Oklahoma waterbody and its contributing watershed. This map also displays the locations of
the water quality monitoring (WQM) stations used as the basis for placement of this waterbody
on the Oklahoma 303(d) list. The waterbody and its surrounding watershed are hereinafter
referred to as the Study Area.
The TMDL established in this report is a necessary step in the process to develop the
turbidity controls needed to restore the fish and wildlife propagation designated for the
waterbody. Table 1-1 provides a description of the locations of the WQM stations on the
303(d)-listed waterbody.
Table 1-1 Water Quality Monitoring Stations used for 2008 303(d) Listing Decision
WQM Station
WQM Station
Location
Description
WQM Station
Location Legal
Descriptions
Latitude Longitude
OK410600010030G Sulphur Creek
SW¼ SW¼ NE¼
Section 16-7S-12E
33.94658 -96.049
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Figure 1-1 Watersheds Not Supporting Fish and Wildlife Propagation Use within the
Study Area
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1.2 Watershed Description
General. The Blue River Basin is located in the southern portion of Oklahoma. The
waterbody addressed in this report is located in Bryan County. The Study Area is located in the
South Central Plains ecoregion of Oklahoma.
Table 1-2, derived from the 2000 U.S. Census, shows that Bryan County where this
watershed is located is sparsely populated (U.S. Census Bureau 2000).
Table 1-2 County Population and Density
County Name
Population
(2000 Census)
Population Density
(per square mile)
Bryan 36,534 40
Climate. Table 1-3 summarizes the average annual precipitation for the waterbody.
Average annual precipitation for the waterbody between 1971 and 2000 was 46.4 inches
(Oklahoma Climate Survey 2005).
Table 1-3 Average Annual Precipitation by Watershed
Sulphur Creek Precipitation Summary
Waterbody Name Waterbody ID
Average
Annual
(inches)
Sulphur Creek OK410600010030_00 46.4
Land Use. Table 1-4 summarizes the acreages and the corresponding percentages of the
land use categories for the contributing watershed associated with the Sulphur Creek
watershed. The land use/land cover data were derived from the U.S. Geological Survey
(USGS) 2001 National Land Cover Dataset (USGS 2007). The land use categories are
displayed in Figure 1-2.
The primary land use category in the Study Area is pasture/hay, which makes up 38
percent of the watershed. The second most common land use within the Study Area is
deciduous forest and grassland at 28 and 27 percent, respectively. Bennington, the only town
within the Sulphur Creek watershed, has an estimated population of 289 (U.S. Census Bureau
2000).
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Table 1-4 Land Use Summaries by Watershed
Landuse Category Sulphur Creek
Waterbody ID OK410600010030_00
Percent Herbaceous Wetlands 0%
Percent Woody Wetlands 0%
Percent Cultivated 1%
Percent Pasture/Hay 38%
Percent Grassland 27%
Percent Shrubland 0%
Percent Mixed Forest 0%
Percent Evergreen Forest 0%
Percent Deciduous Forest 28%
Percent Barren 0%
Percent Developed - High Intensity 0%
Percent Developed - Medium Intensity 0%
Percent Developed - Low Intensity 0%
Percent Developed - Open 5%
Percent Water 1%
Acres Herbaceous Wetlands 5
Acres Woody Wetlands 0
Acres Cultivated 165
Acres Pasture/Hay 7,924
Acres Grassland 5,513
Acres Shrubland 0
Acres Mixed Forest 0
Acres Evergreen Forest 12
Acres Deciduous Forest 5,897
Acres Barren 2
Acres Developed - High Intensity 0
Acres Developed - Medium Intensity 9
Acres Developed - Low Intensity 43
Acres Developed - Open 1,011
Acres Water 112
Total (Acres) 20,693
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Figure 1-2 Land Use Map by Watershed
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1.3 Stream Flow Data
Stream flow characteristics and data are key information when conducting water quality
assessments such as TMDLs. While the USGS operates flow gages throughout Oklahoma,
there is no flow gage located on Sulphur Creek. Some flow measurements were collected at
the same time TSS and turbidity water quality samples were collected at various WQM
stations. These data are included in Appendix A along with turbidity and TSS data.
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SECTION 2
PROBLEM IDENTIFICATION AND WATER QUALITY TARGET
2.1 Oklahoma Water Quality Standards
Title 785 of the Oklahoma Administrative Code authorizes the Oklahoma Water Resources
Board (OWRB) to promulgate Oklahoma’s water quality standards and implementation
procedures (OWRB 2008). The OWRB has statutory authority and responsibility concerning
establishment of state water quality standards, as provided under 82 Oklahoma Statute [O.S.],
§1085.30. This statute authorizes the OWRB to promulgate rules …which establish
classifications of uses of waters of the state, criteria to maintain and protect such
classifications, and other standards or policies pertaining to the quality of such waters. [O.S.
82:1085:30(A)]. Beneficial uses are designated for all waters of the state. Such uses are
protected through restrictions imposed by the antidegradation policy statement, narrative water
quality criteria, and numerical criteria (OWRB 2008). The beneficial uses designated for
Sulphur Creek (OK410600010030_00) include primary body contact recreation, warm water
aquatic community, fish consumption, agriculture and aesthetics. The TMDL in this report
addresses fish and wildlife propagation beneficial use for the subcategory warm water aquatic
community. Table 2-1, an excerpt from Appendix B of the 2008 Integrated Report
(ODEQ 2008), summarizes the warm water aquatic community use attainment status and the
scheduled date for TMDL development established by ODEQ for the impaired waterbody of
the Study Area. The 2008 Integrated Report (ODEQ 2008) identifies the Sulphur Creek as
Priority 2 for TMDL development. Priority 2 waterbodies are targeted for TMDL development
by 2013. The TMDL established in this report is a necessary step in the process to restore the
fish and wildlife propagation designation for this waterbody.
The numeric criteria for turbidity to maintain and protect the use of “Fish and Wildlife
Propagation” from Title 785:45-5-12 (f) (7) is as follows:
(A) Turbidity from other than natural sources shall be restricted to not exceed the following
numerical limits:
4. Cool Water Aquatic Community/Trout Fisheries: 10 NTUs;
5. Lakes: 25 NTU; and
6. Other surface waters: 50 NTUs.
(B) In waters where background turbidity exceeds these values, turbidity from point sources
will be restricted to not exceed ambient levels.
(C) Numerical criteria listed in (A) of this paragraph apply only to seasonal base flow
conditions.
(D) Elevated turbidity levels may be expected during, and for several days after, a runoff event.
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Table 2-1 Excerpt from the 2008 Integrated Report – Comprehensive Waterbody
Assessment Category List
Waterbody ID Waterbody Name
Stream Miles
Category
TMDL Date
Priority
Warm Water
Aquatic
Community
OK410600010030_00 Sulphur Creek 14.6 5a 2013 2 N
N = Not Supporting;
5a = TMDL is underway or will be scheduled
Source: 2008 Integrated Report, ODEQ 2008
To implement Oklahoma’s WQS for Fish and Wildlife Propagation, OWRB promulgated
Chapter 46, Implementation of Oklahoma’s Water Quality Standards (OWRB 2008). The
excerpt below from Chapter 46: 785:46-15-5, stipulates how water quality data will be assessed
to determine support of fish and wildlife propagation as well as how the water quality target for
TMDLs will be defined for turbidity.
Assessment of Fish and Wildlife Propagation support
(a) Scope. The provisions of this Section shall be used to determine whether the beneficial
use of Fish and Wildlife Propagation or any subcategory thereof designated in OAC 785:45 for
a waterbody is supported.
(e) Turbidity. The criteria for turbidity stated in 785:45-5-12(f)(7) shall constitute the
screening levels for turbidity. The tests for use support shall follow the default protocol in
785:46-15-4(b).
785:46-15-4. Default protocols
(b) Short term average numerical parameters.
(1) Short term average numerical parameters are based upon exposure periods of less than
seven days. Short term average parameters to which this Section applies include, but are not
limited to, sample standards and turbidity.
(2) A beneficial use shall be deemed to be fully supported for a given parameter whose
criterion is based upon a short term average if 10% or less of the samples for that parameter
exceed the applicable screening level prescribed in this Subchapter.
(3) A beneficial use shall be deemed to be fully supported but threatened if the use is
supported currently but the appropriate state environmental agency determines that available
data indicate that during the next five years the use may become not supported due to
anticipated sources or adverse trends of pollution not prevented or controlled. If data from the
preceding two year period indicate a trend away from impairment, the appropriate agency
shall remove the threatened status.
(4) A beneficial use shall be deemed to be not supported for a given parameter whose
criterion is based upon a short term average if at least 10% of the samples for that parameter
exceed the applicable screening level prescribed in this Subchapter.
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2.2 Problem Identification
Turbidity is a measure of water clarity and is caused by suspended particles in the water
column. Because turbidity cannot be expressed as a mass load, total suspended solids (TSS)
are used as a surrogate in this TMDL. Therefore, both turbidity and TSS data are presented in
this section.
Table 2-2 summarizes water quality data collected from the WQM stations between 1991
and 2007 for turbidity. However, as stipulated in Title 785:45-5-12 (f) (7) (C), numeric criteria
for turbidity only apply under base flow conditions. While the base flow condition is not
specifically defined in the Oklahoma Water Quality Standards, DEQ considers base flow
conditions to be all flows less than the 25% flow exceedance frequency (i.e., the lower 75
percent of flows) which is consistent with the USGS Streamflow Conditions Index (USGS
2009). Therefore, Table 2-3 was prepared to represent the subset of these data for samples
collected during base flow conditions. Water quality samples collected under flow conditions
greater than the 25% flow exceedance frequency were therefore excluded from the data set
used for TMDL analysis. The data in Table 2-3 were used to support the decision to place
Sulphur Creek on the ODEQ 2008 303(d) list (ODEQ 2008) for nonsupport of the Fish and
Wildlife Propagation use based on turbidity levels observed in the waterbody.
Table 2-2 Summary of All Turbidity Samples 1991 - 2007
WQM Station
Number of
Turbidity
Samples
Number of
Samples Exceed 50
Nephelometric
Turbidity Units
(NTU)
Percentage of
Samples
Exceeding
Criterion
Average Turbidity
(NTU) Per WQM
Station
OK410600010030G 22 5 23% 82
Table 2-3 Summary of Turbidity Samples Collected During Base Flow Conditions
1992 - 2007
WQM Station
Number of Turbidity
Samples Collected
During Base Flow
Conditions
Number of
Samples
Exceeding 50 NTU
during Base Flow
Conditions
Percentage of
Samples
Exceeding
Criterion
during Base
Flow
Conditions
Average Turbidity
(NTU) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 19 2 11% 34
Table 2-4 summarizes water quality data collected from the WQM stations between 1991
and 2007 for TSS. Table 2-5 presents a subset of these data for samples collected during base
flow conditions. Water quality data for turbidity and TSS are provided in Appendix A.
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Table 2-4 Summary of All TSS Samples 1991 - 2007
WQM Station
Number of TSS
Samples
Average TSS
(mg/L) Per WQM
Station
OK410600010030G 21 76
Table 2-5 Summary of TSS Samples Collected During Base Flow Conditions
1992 -2007
WQM Station
Number of TSS
Samples Collected
During Base Flow
Conditions
Average TSS
(mg/L) Per WQM
Station during
Base Flow
Conditions
OK410600010030G 18 19
2.3 Water Quality Target
The Code of Federal Regulations (40 CFR §130.7(c)(1)) states that, “TMDLs shall be
established at levels necessary to attain and maintain the applicable narrative and numerical
water quality standards.” An individual water quality target established for turbidity must
demonstrate compliance with the numeric criteria prescribed in the Oklahoma WQS
(OWRB 2008). According to the Oklahoma WQS [785:45-5-12(f)(7)], the turbidity criterion
for streams with warm water aquatic community (WWAC) beneficial use is 50 NTUs (OWRB
2008). The turbidity of 50 NTUs applies only to seasonal base flow conditions. Turbidity
levels are expected to be elevated during, and for several days after, a storm event.
TMDLs for turbidity in streams designated as warm water aquatic community must take
into account that no more than 10 percent of the samples may exceed the numeric criterion of
50 NTU. However, as described above, because turbidity cannot be expressed as a mass load,
TSS is used as a surrogate in this TMDL. Since there is no numeric criterion in the Oklahoma
WQS for TSS, a specific method must be developed to convert the turbidity criterion to TSS
based on a relationship between turbidity and TSS. The method for deriving the relationship
between turbidity and TSS and for calculating a water body specific water quality target using
TSS is summarized in Section 4 of this report.
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SECTION 3
POLLUTANT SOURCE ASSESSMENT
A pollutant source assessment characterizes known and suspected sources of pollutant
loading to impaired waterbodies. Sources within a watershed are categorized and quantified to
the extent that information is available. Turbidity may originate from NPDES-permitted
facilities, fields, construction sites, quarries, stormwater runoff and eroding stream banks.
Point sources are permitted through the NPDES program. NPDES-permitted facilities that
discharge treated wastewater are required to monitor for TSS in accordance with their permit.
Nonpoint sources are diffuse sources that typically cannot be identified as entering a waterbody
through a discrete conveyance at a single location. These sources may involve land activities
that contribute TSS to surface water as a result of rainfall runoff. For the TMDL in this report,
all sources of pollutant loading not regulated by NPDES permits are considered nonpoint
sources.
The 2008 Integrated Water Quality Assessment Report (ODEQ 2008) listed potential
sources of turbidity in Sulphur Creek (OK410600010030_00) as grazing in riparian corridors of
streams and creeks, highway/road/bridge runoff (non-construction related), non-irrigated crop
production, rangeland grazing, and other unknown sources.
3.1 NPDES-Permitted Facilities
Under 40CFR, §122.2, a point source is described as a discernable, confined, and discrete
conveyance from which pollutants are or may be discharged to surface waters. NPDES-permitted
facilities can be characterized as continuous or stormwater related discharges.
NPDES-permitted facilities classified as point sources include:
NPDES municipal wastewater treatment plant (WWTP);
NPDES Industrial WWTP Discharges;
NPDES municipal separate storm sewer discharge (MS4);
NPDES Concentrated Animal Feeding Operation (CAFO);
NPDES multi-sector general permits; and
NPDES construction stormwater discharges.
Continuous point source discharges from municipal and industrial WWTPs, could result in
discharge of elevated concentrations of TSS if a facility is not properly maintained, is of poor
design, or flow rates exceed capacity. However, in most cases suspended solids discharged by
WWTPs consist primarily of organic solids rather than inorganic suspended solids (i.e., soil and
sediment particles from erosion or sediment resuspension). Discharges of organic suspended
solids from WWTPs are addressed by ODEQ through its permitting of point sources to
maintain WQS for dissolved oxygen. and are not considered a potential source of turbidity in
this TMDL report. Discharges of TSS will be considered to be organic suspended solid if the
discharge permit includes a limit for BOD or CBOD. Only WWTP discharges of inorganic
suspended solids will be considered and will receive wasteload allocations.
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Stormwater runoff from MS4 areas, facilities under multi-sector general permits, and
NPDES construction stormwater discharges, which are regulated under the USEPA NPDES
Program, can contain TSS concentrations. 40 C.F.R. § 130.2(h) requires that NPDES-regulated
storm water discharges must be addressed by the wasteload allocation component of a TMDL.
However, any stormwater discharge by definition occurs during or immediately following
periods of rainfall and elevated flow conditions when where Oklahoma Water Quality Standard
for turbidity does not apply. Oklahoma Water Quality Standards specify that the criteria for
turbidity “apply only to seasonal base flow conditions” and go on to say “Elevated turbidity
levels may be expected during, and for several days after, a runoff event” [OAC 785:45-5-
12(f)(7)]. In other words, the turbidity impairment status is limited to base flow conditions and
stormwater discharges from MS4 areas or construction sites do not contribute to the violation
of Oklahoma’s turbidity standard. Therefore, WLA for NPDES-regulated storm water
discharges is essentially considered unnecessary in this TMDL report and will not be included
in the TMDL calculations.
3.1.1 Continuous Point Source Discharges
There are no municipal or industrial NPDES-permitted facilities within the Study Area.
3.1.2 Concentrated Animal Feeding Operations
There are no CAFOs within the Study Area.
3.1.3 Stormwater Permits for MA4 and Construction Activities
There are no urbanized areas designated as MS4s within this Study Area. A general
stormwater permit is required for construction activities. Permittees are authorized to discharge
pollutants in stormwater runoff associated with construction activities for construction sites.
Stormwater discharges occur only during or immediately following periods of rainfall and
elevated flow conditions when the turbidity criteria do not apply and are not considered
potential contributors to turbidity impairment.
3.1.4 Section 404 permits
Section 404 of the Clean Water Act (CWA) establishes a program to regulate the discharge
of dredged or fill material into waters of the United States, including wetlands. Activities in
waters of the United States regulated under this program include fill for development, water
resource projects (such as dams and levees), infrastructure development (such as highways and
airports) and mining projects. Section 404 requires a permit before dredged or fill material may
be discharged into waters of the United States, unless the activity is exempt from Section 404
regulation (e.g. certain farming and forestry activities).
Section 404 permits are administrated by the U.S. Army Corps of Engineers. EPA reviews
and provides comments on each permit application to make sure it adequately protects water
quality and complies with applicable guidelines. Both USACE and EPA can take enforcement
actions for violations of Section 404.
Discharge of dredged or fill material in waters can be a significant source of turbidity/TSS.
The federal Clean Water Act requires that a permit be issued for activities which discharge
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dredged or fill materials into the waters of the United States, including wetlands. The state of
Oklahoma will use its Section 401 certification authority to ensure Section 404 permits protect
oklahoma water quality standards.
3.2 Nonpoint Sources
Nonpoint sources include those sources that cannot be identified as entering the waterbody
at a specific location. The relatively homogeneous land use/land cover categories within the
Study Area are associated with agricultural and range management activities. This suggests
that potential nonpoint sources of TSS include sediments originating from grazing in riparian
corridors of streams and creeks, highway/road/bridge runoff (non-construction related), non-irrigated
crop production, rangeland grazing and other sources of sediment loading
(ODEQ 2008). Elevated turbidity measurements can be caused by stream bank erosion
processes, stormwater runoff events and channel disturbances. However, there is insufficient
data available to quantify contributions of TSS from these processes. TSS or sediment loading
can also occur under non-runoff conditions as a result of anthropogenic activities in riparian
corridors which cause erosive conditions. Sediment loading of streams can also originate from
natural erosion processes, including the weathering of soil, rocks, and uncultivated land;
geological abrasion; and other natural phenomena. Given the lack of data to establish the
background conditions for TSS/turbidity, separating background loading from nonpoint sources
is not feasible in this TMDL development.
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Figure 3-1 Locations of Permitted Facilities in the Study Area
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SECTION 4
TECHNICAL APPROACH AND METHODS
The objective of a TMDL is to estimate allowable pollutant loads and to allocate these
loads to the known pollutant sources in the watershed so appropriate control measures can be
implemented and the WQS achieved. A TMDL is expressed as the sum of three elements as
described in the following mathematical equation:
TMDL = Σ WLA + Σ LA + MOS
The WLA is the portion of the TMDL allocated to existing and future point sources. The
LA is the portion of the TMDL allocated to nonpoint sources, including natural background
sources. The MOS is intended to ensure that WQS will be met. Thus, the allowable pollutant
load that can be allocated to point and nonpoint sources can then be defined as the TMDL
minus the MOS.
4.1 Determining a Surrogate Target
40 CFR, §130.2(1), states that TMDLs can be expressed in terms of mass per time,
toxicity, or other appropriate measures. Turbidity is a commonly measured indicator of the
suspended solids load in streams. However, turbidity is an optical property of water, and
measures scattering of light by suspended solids and colloidal matter. To develop TMDLs, a
gravimetric (mass-based) measure of solids loading is required to express loads. There is often
a strong relationship between the total suspended solids concentration and turbidity. Therefore,
the TSS load, which is expressed as mass per time, is used as a surrogate for turbidity and
represents the maximum one-day load the stream can assimilate while still attaining the WQS.
To determine the relationship between turbidity and TSS, a linear regression between TSS
and turbidity was developed using data collected from 1991 to 2007 at one station within the
Study Area. Prior to developing the regression the following steps were taken to refine the
dataset:
Assign values to censored data (i.e., measured concentrations lower than the analytical
quantitation limit and, thus, reported as less than the quantitation limit). For example,
using 9.99 to replace samples reported as “<10”;
Check rainfall data on the day when samples were collected and on the previous two
days. If there was a significant rainfall event (>= 1.0 inch) in any of these days, the
sample will be excluded from regression analysis with one exception. If the significant
rainfall happened on the sampling day and the turbidity reading was less than 25 NTUs
(half of turbidity standard for streams), the sample will not be excluded from analysis
because most likely the rainfall occurred after the sample was taken;
Remove data collected under high flow conditions exceeding the base-flow criterion.
This means that measurements corresponding to flow exceedance frequencies lower
than 25% were not used in the regression; and
Log-transform both turbidity and TSS data to minimize effects of their non-linear data
distributions.
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When ordinary least squares regression (OLS) is applied to ascertain the best relationship
between two variables (i.e., X and Y), one variable (Y) is considered “dependent” on the other
variable (X), but X must be considered “independent” of the other, and known without
measurement error. OLS minimizes the differences, or residuals, between measured Y values
and Y values predicted based on the X variable.
For current purposes, a relationship is necessary to predict TSS concentrations from
measured turbidity values, but also to translate the TSS-based TMDL back to in-stream
turbidity values. For this purpose, an alternate regression fitting procedure known as the line of
organic correlation (LOC) was applied. The LOC has three advantages over OLS (Helsel and
Hirsch 2002):
LOC minimizes fitted residuals in both the X and Y directions;
It provides a unique best-fit line regardless of which parameter is used as the
independent variable; and
Regression-fitted values have the same variance as the original data.
The LOC minimizes the areas of the right triangles formed by horizontal and vertical lines
drawn from observations to the fitted line. The slope of the LOC line equals the geometric
mean of the Y on X (TSS on turbidity) and X on Y (turbidity on TSS) OLS slopes, and is
calculated as:
x
y
s
s
m1 m m' sign[r]
where m1 is the slope of the LOC line, m is the TSS on turbidity OLS slope, m’ is the turbidity
on TSS OLS slope, r is the TSS-turbidity correlation coefficient, sy is the standard deviation of
the TSS measurements, and sx is the standard deviation of the turbidity measurements.
The intercept of the LOC (b1) is subsequently found by fitting the line with the LOC slope
through the point (mean turbidity, mean TSS). The correlation between TSS and turbidity,
along with the LOC and the OLS lines are shown in Figure 4-1.
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Figure 4-1 Linear Regression for TSS-Turbidity under Base-flow Conditions for
Sulphur Creek (OK 410600010030_00)
The normalized root mean square error (NRMSE) and R-square (R2) were used as the
primary measures of goodness-of-fit. As shown in Figure 4-1, the LOC yields a NRMSE value
of 14.9 which means the root mean square error (RMSE) is 14.9% of the average of the
measured TSS values. The R-square (R2) value indicates the fraction of the total variance in
TSS or turbidity observations that is explained by the LOC.
It was noted that there were a few outliers that exerted undue influence on the regression
relationship. These outliers were identified by applying the Tukey’s Boxplot method
(Tukey 1977) to the dataset of the distances from observed points to the regression line. The
Tukey Method is based on the interquartile range (IQR), the difference between the 75th
percentile (Q3) and 25th percentile (Q1) of distances between observed points and the LOC.
Using the Tukey method, any point with an error greater than Q3 + 1.5* IQR or less than Q1 –
1.5*IQR was identified as an outlier and removed from the regression dataset. The above
regressions were calculated using the dataset with outliers removed.
The Tukey Method is equivalent to using three times the standard deviation to identify
outliers if the residuals (observed - predicted) follow a normal distribution. The probability of
sampling results being within three standard deviations of the mean is 99.73% while the
probability for the Tukey Method is 99.65%. If three times the standard deviation is used to
identify outliers, it is necessary to first confirm that the residuals are indeed normally
distributed. This is difficult to do because of the size limitations of the existing turbidity &
TSS dataset. Tukey’s method does not rely on any assumption about the distribution of the
residuals. It can be used regardless of the shape of distribution.
Using the regression equation shown in Figure 4-1, a turbidity value of 50 NTU (standard
applicable to Sulphur Creek) corresponds to a TSS concentration of 31.4 mg/L.
1
10
100
1000
1 10 100 1000
TSS (mg/L)
Turbidity (NTU)
log(TSS) = 0.7342*log(Turb) +
0.2489
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4.2 Using Load Duration Curves to Develop TMDLs
The TMDL calculations presented in this report are derived from load duration curves
(LDC). LDCs facilitate rapid development of TMDLs, and as a TMDL development tool,
indicate whether impairments are associated with point or nonpoint sources. The technical
approach for using LDCs for TMDL development includes the four following steps described
in Subsections 4.3 through 4.4 below:
Preparing flow duration curves for gaged and ungaged WQM stations;
Estimating loading in the receiving water using measured TSS water quality data and
turbidity-converted data;
Determining the overall percent reduction goal (PRG) necessary to attain WQS; and
Historically, in developing WLAs for pollutants from point sources, it was customary to
designate a critical low flow condition (e.g., 7Q2) at which the maximum permissible loading
was calculated. As water quality management efforts expanded in scope to quantitatively
address nonpoint sources of pollution and various types of pollutants, it became clear that this
single critical low flow condition was inadequate to ensure adequate water quality across a
range of flow conditions. Use of the LDC obviates the need to determine a design storm or
selected flow recurrence interval with which to characterize the appropriate flow level for the
assessment of critical conditions. For waterbodies impacted by both point and nonpoint
sources, the “nonpoint source critical condition” would typically occur during high flows, when
rainfall runoff would contribute the bulk of the pollutant load, while the “point source critical
condition” would typically occur during low flows, when point source discharges would
dominate the base flow of the impaired water. However, flow range is only a general indicator
of the relative proportion of point/nonpoint contributions. It is not used in this report to
quantify point source or nonpoint source contributions. Violations that occur during low flows
may not be caused exclusively by point sources. Violations have been noted in some
watersheds that contain no point sources.
LDCs display the maximum allowable load over the complete range of flow conditions by
a line using the calculation of flow multiplied by the water quality criterion. The TMDL can be
expressed as a continuous function of flow, equal to the line, or as a discrete value derived from
a specific flow condition.
4.3 Development of Flow Duration Curves
Flow duration curves serve as the foundation of LDCs and are graphical representations of
the flow characteristics of a stream at a given site. Flow duration curves utilize the historical
hydrologic record from stream gages to forecast future recurrence frequencies. Many WQM
stations throughout Oklahoma do not have long-term flow data; therefore, flow frequencies
must be estimated. The most basic method to estimate flows at an ungaged site involves
1) identifying a downstream flow gage; 2) calculating the contributing drainage areas of the
ungaged sites and the flow gage; and 3) calculating daily flows at the ungaged site by using the
flow at the gaged site multiplied by the drainage area ratio. A more complex approach used to
support this analysis also considers watershed differences in rainfall, land use, and the
hydrologic properties of soil that govern runoff and retention. For the Sulphur Creek
watershed, flows were projected using data from USGS 07332500, Blue River near Blue, OK.
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A more detailed explanation of the methods for estimating flow at ungaged WQM stations is
provided in Appendix B.
Flow duration curves are a type of cumulative distribution function. The flow duration
curve represents the fraction of flow observations that equal or exceed a given flow at the site
of interest. The observed flow values are first ranked from highest to lowest then, for each
observation, the percentage of observations equal to or exceeding that flow is calculated. The
flow value is read from the ordinate (y-axis), which is typically on a logarithmic scale since the
high flows would otherwise overwhelm the low flows. The flow exceedance frequency is read
from the abscissa (x-axis), which is numbered from 0 to 100 percent, and may or may not be
logarithmic. The flow exceedance frequency is defined as percent of time a given flow was
equaled or exceeded based on daily flow values. Therefore, the lowest measured flow occurs at
an exceedance frequency of 100 percent indicating that flow has equaled or exceeded this value
100 percent of the time, while the highest measured flow is found at an exceedance frequency
of 0 percent. The median flow occurs at a flow exceedance frequency of 50 percent. The flow
exceedance frequencies of the USGS gage used to project flows for this report are provided in
Appendix B.
While the number of observations required to develop a flow duration curve is not
rigorously specified, a flow duration curve is usually based on more than 1 year of
observations, and encompasses inter-annual and seasonal variation. Ideally, the drought of
record and flood of record are included in the observations. For this purpose, the long-term
flow gaging stations operated by the USGS are utilized (USGS 2007a).
A typical semi-log flow duration curve exhibits a sigmoidal shape, bending upward near a
flow exceedance frequency value of 0 percent and downward at a frequency near 100 percent,
often with a relatively constant slope in between. For sites that on occasion exhibit no flow, the
curve will intersect the x-axis at a frequency less than 100 percent. As the number of
observations at a site increases, the line of the LDC tends to appear smoother. However, at
extreme low and high flow values, flow duration curves may exhibit a “stair step” effect due to
the USGS flow data rounding conventions near the limits of quantitation.
Flow duration curves are generated using a DEQ automated application referred to as the
Oklahoma TMDL toolbox. Figure 4-2 shows the flow duration curve generated from the
Oklahoma TMDL toolbox for Sulphur Creek using flow data from 1984 to 2006. The USGS
National Water Information System serves as the primary source of flow measurements for the
application. All available daily average flow values for all gages in Oklahoma, as well as the
nearest upstream and downstream gages in adjacent states, were retrieved for use in the
application. The application includes a data update module that automatically downloads the
most recent USGS data and appends it to the existing flow database.
Some instantaneous flow measurements were available from various agencies. These were
not combined with the daily average flows or used in calculating flow percentiles, but were
matched to TSS and/or turbidity grab measurements collected at the same site and time. When
available, these instantaneous flow measurements were used in lieu of the daily average flow to
calculate instantaneous TSS loads.
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Figure 4-2 Flow Duration Curve for Sulphur Creek (OK410600010030_00)
4.4 Development of TMDLs Using Load Duration Curves
The final step in the TMDL calculation process involves a group of additional
computations derived from the preparation of LDCs. These computations are necessary to
derive a PRG (which is one method of presenting how much TSS loading must be reduced to
meet turbidity WQS in the impaired watershed).
Step 1: Generate LDCs. LDCs are similar in appearance to flow duration curves;
however, the ordinate is expressed in terms of a load typically in lbs/day. The curve represents
the water quality target for TSS (31 mg/L) expressed in terms of a load through multiplication
by the continuum of flows historically observed at this site. The basic steps to generating an
LDC involve:
obtaining daily flow data for the site of interest from the USGS (or project flow using
Oklahoma TMDL Toolbox if station is ungaged);
sorting the flow data and calculating flow exceedance frequencies for the time period
and season of interest;
obtaining available turbidity and TSS water quality data;
matching the water quality observations with the flow data from the same date;
displaying a curve on a plot that represents the allowable load multiplying the actual or
estimated flow by the WQtarget for TSS;
multiplying the flow by the water quality parameter concentration to calculate daily
loads (for sampling events with both TSS and turbidity data, the measured TSS value
is used; if only turbidity was measured, the value was converted to TSS using the
regression equation in Figure 4-1); then
0.1
1.0
10.0
100.0
1000.0
10000.0
0 10 20 30 40 50 60 70 80 90 100
Flow (cfs)
Flow Exceedence Frequency (%)
Flow Duration Curver (OK410600010030_00)
High flow conditions
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plotting the flow exceedance frequencies and daily load observations in a load duration
plot.
The culmination of these steps is expressed in the following formula, which is displayed on
the LDC as the TMDL curve:
TMDL (lb/day) = WQtarget * flow (cfs) * unit conversion factor
where: WQtarget = 31.4 mg/L (TSS)
unit conversion factor = 5.39377 L*s*lb /(ft3*day*mg)
The flow exceedance frequency (x-value of each point) is obtained by looking up the
historical exceedance frequency of the measured or estimated flow; in other words, the percent
of historical observations that equal or exceed the measured or estimated flow. Historical
observations of TSS and/or turbidity concentrations are paired with flow data and are plotted on
the LDC. The TSS load (or the y-value of each point) is calculated by multiplying the TSS
concentration (measured or converted from turbidity) (mg/L) by the instantaneous flow (cfs) at
the same site and time, with appropriate volumetric and time unit conversions. TSS loads
representing exceedance of water quality criteria fall above the TMDL line.
As noted earlier, runoff has a strong influence on loading of nonpoint source pollution yet
flows do not always correspond directly to local runoff. High flows may occur in dry weather
due to upstream precipitation events or releases form upstream dams. Runoff influence may be
observed with low or moderate flows depending on antecedent conditions.
Step 2: Define MOS. The MOS may be defined explicitly or implicitly. A typical
explicit approach would reserve some specific fraction of the TMDL as the MOS. In an
implicit approach, conservative assumptions used in developing the TMDL are relied upon to
provide an MOS to assure that WQSs are attained. For turbidity (TSS) TMDLs an explicit
MOS is derived from the NRMSE established by the turbidity/TSS regression analysis
conducted for each waterbody. This approach for setting an explicit MOS has been used in
other approved turbidity TMDLs
For the TMDLs in this report, an explicit MOS of 10 percent was selected.
Step 3: Calculate WLA. As previously stated, the pollutant load allocation for point
sources is defined by the WLA. For TMDL development purposes when addressing turbidity
or TSS, a WLA will be established for wastewater (continuous) discharges in impaired
watersheds that do not have a BOD or CBOD permit limit but do have a TSS limit. These point
source discharges of inorganic suspended solids will be assigned a TSS WLA as part of
turbidity TMDLs to ensure WQS can be maintained.
The LDC approach recognizes that the assimilative capacity of a waterbody depends on the
flow, and that maximum allowable loading will vary with flow condition. TMDLs can be
expressed in terms of maximum allowable concentrations, or as different maximum loads
allowable under different flow conditions, rather than single maximum load values. A load-based
approach meets the requirements of 40 CFR, 130.2(i) for expressing TMDLs “in terms of
mass per time, toxicity, or other appropriate measures.”
WLA for WWTP. WLAs may be set to zero for watersheds with no existing or planned
continuous permitted point sources such as Sulphur Creek.
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WLA for Permitted Stormwater. For turbidity TMDLs, WLAs for permitted stormwater
such as MS4s, construction, and multi-sector general permits are not calculated since these
discharges occur under high flow conditions when the turbidity criteria do not apply.
Step 4: Calculate LA. Given the lack of data and the variability of storm events, it is
difficult to quantify discharges that accurately represent projected loadings from nonpoint
sources. LAs can be calculated under different flow conditions as the water quality target load
minus the WLA. The LA is represented by the area under the LDC but above the WLA. The
LA at any particular flow exceedance is calculated as shown in the equation below.
LA = TMDL - WLA - MOS
Step 5: Estimate LA Load Reduction. After existing loading estimates are computed,
nonpoint load reduction estimates are calculated by using the difference between estimated
existing loading and the allowable load expressed by the LDC (TMDL-MOS). This difference
is expressed as the overall PRG for the impaired waterbody. For turbidity, the PRG is the load
reduction that ensures that no more than 10 percent of the samples under flow-base conditions
exceed the TMDL.
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SECTION 5
TMDL CALCULATIONS
5.1 Estimated Loading and Critical Conditions
USEPA regulations at 40 CFR 130.7(c) (1) require TMDLs to take into account critical
conditions for stream flow, loading, and all applicable water quality standards. To accomplish
this, available instream WQM data were evaluated with respect to flows and magnitude of
water quality criteria exceedance using LDCs.
To calculate the TSS load at the WQtarget, the flow rate at each flow exceedance frequency
is multiplied by a unit conversion factor (5.39377 L*s*lb /ft3/day/mg) and the TSS target
(31 mg/L). This calculation produces the maximum TSS load in the stream that will result in
attainment of the 50 NTU standard for turbidity. The allowable TSS loads at the WQS
establish the TMDL and are plotted versus flow exceedance frequency as a LDC. The x-axis
indicates the flow exceedance frequency, while the y-axis is expressed in terms of a TSS load
in pounds per day.
To estimate existing loading, TSS and turbidity observations from 1991 to 2007 are paired
with the flows measured or estimated in that segment on the same date. For sampling events
with both TSS and turbidity data, the measured TSS value is used; if only turbidity was
measured, the value was converted to TSS using the regression equation in Figure 4-1.
Pollutant loads are then calculated by multiplying the TSS concentration by the flow rate and
the unit conversion factor. The associated flow exceedance frequency is then matched with the
measured flow from the tables provided in Appendix B. The observed TSS or converted
turbidity loads are then added to the LDC plot as points. These points represent individual
ambient water quality samples of TSS. Points above the LDC indicate the TSS target was
exceeded at the time of sampling. Conversely, points under the LDC indicate the sample did
not exceed the WQtarget. Figure 5-1 shows the LDC developed for Sulphur Creek. It is noted
that the LDC plot includes data under all flow conditions to show the overall condition of the
stream. However, it is noted that the turbidity standard only applies for base-flow conditions.
Thus, when assessing beneficial use assessment, only the portion of the graph corresponding to
flows from the 25% to 100% flow exceedance frequency should be used.
The LDC approach recognizes that the assimilative capacity of a waterbody depends on the
flow, and that maximum allowable loading varies with flow condition. Existing loading, and
load reductions required to meet the TMDL water quality target can also be calculated under
different flow conditions. The difference between existing loading and the water quality target
is used to calculate the loading reductions required. The overall PRG is calculated for Sulphur
Creek as the reduction in load required so no more than 10 percent of the samples collected
under base-flow conditions would exceed 28.3 mg/L (90 percent of the TSS WQtarget to account
for the explicit MOS). This is done through an iterative process of taking a series of percent
reduction values applying each value uniformly between the concentrations of samples and
verifying that no more than 10 percent of the samples exceed the water quality target
concentration. The concentrations are derived from only those samples after high flow samples
are excluded. The PRG for Sulphur Creek is estimated to be 11.7 percent.
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Figure 5-1 Load Duration Curve for Total Suspended Solids in Sulphur Creek
(OK410600010030_00)
As shown in Figure 5-1, TSS levels exceed the water quality target less than 20% of the
time.
5.2 Wasteload Allocation
The WLA_WWTF for the Study Area is zero.
No wasteload allocations are needed for stormwater dischargers. By definition, any
stormwater discharge occurs during periods of rainfall and elevated flow conditions.
Oklahoma’s Water Quality Standards specify that the criteria for turbidity “apply only to
seasonal base flow conditions” and go on to say “Elevated turbidity levels may be expected
during, and for several days after, a runoff event” [OAC 785:45-5-12(f)(7)]. Therefore, WLA
for NPDES-regulated storm water discharges is essentially considered unnecessary in this
TMDL report and will not be included in the TMDL calculations. Conditions in existing
stormwater permits are sufficient to protect receiving waters.
To accommodate the potential for future growth in the watershed, 1% of TSS loading is
reserved as part of the WLA.
5.2.1 Section 404 permits
No TSS wasteload allocations were set aside for Section 404 permits. The state will use its
Section 401 certification authority to ensure Section 404 permits protect Oklahoma water
quality standards and comply with TSS TMDLs in this report. Section 404 permits will be
conditioned to meet one of the following two conditions to be certified by the state:
1.0
10.0
100.0
1000.0
10000.0
100000.0
1000000.0
10000000.0
0 10 20 30 40 50 60 70 80 90 100
TSS Load (lbs/day)
Flow Exceedance Frequency (%)
Load Duration Curver (OK410600010030_00)
High flow conditions
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Include TSS limits in the permit and establish a monitoring requirement to ensure
compliance with turbidity standard and TSS TMDLs.
Submit to the ODEQ a BMP turbidity reduction plan which should include all
practicable turbidity control techniques. The turbidity reduction plan must be approved
first before a Section 404 permit can be issued.
5.3 Load Allocation
As discussed in Section 3.2, pollutant loading to the receiving streams of each waterbody
emanate from a number of different nonpoint sources. The data analysis and the LDCs
demonstrate that exceedances of the turbidity WQS at the WQM stations are the result of a
variety of nonpoint sources. The LA is calculated as the difference between the TMDL, MOS,
and WLA as follows:
LA = TMDL – WLA_WWTP – WLA_growth - MOS
5.4 Seasonal Variability
Federal regulations (40 CFR §130.7(c)(1)) require that TMDLs account for seasonal
variation in watershed conditions and pollutant loading. The TMDL established in this report
adhere to the seasonal application of the Oklahoma WQS for turbidity, which applies to
seasonal base flow conditions only. Seasonal variation was also accounted for in this TMDL
by using more than 5 years of water quality data and by using the longest period of USGS flow
records possible when estimating flows to develop flow exceedance frequency.
5.5 Margin of Safety
Federal regulations (40 CFR §130.7(c)(1)) require that TMDLs include an MOS. The
MOS is a conservative measure incorporated into the TMDL equation that accounts for the lack
of knowledge associated with calculating the allowable pollutant loading to ensure WQSs are
attained. USEPA guidance allows for use of implicit or explicit expressions of the MOS, or
both. When conservative assumptions are used in development of the TMDL, or conservative
factors are used in the calculations, the MOS is implicit. When a specific percentage of the
TMDL is set aside to account for lack of knowledge, then the MOS is considered explicit.
An explicit Margin of Safety of 10% was selected in this TMDL report.
5.6 TMDL Calculations
This TMDL was derived using the LDC method. A TMDL is expressed as the sum of all
WLAs (point source loads), LAs (nonpoint source loads), and an appropriate MOS, which
attempts to account for lack of knowledge concerning the relationship between effluent
limitations and water quality.
This definition can be expressed by the following equation:
TMDL = Σ WLA + Σ LA + MOS
The TMDL represents a continuum of desired load over all flow conditions, rather than
fixed at a single value, because loading capacity varies as a function of the flow present in the
stream. The higher the flow is, the more wasteload the stream can handle without violating
water quality standards.
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Regardless of the magnitude of the WLA calculated in these TMDLs, future new
discharges or increased load from existing discharges will be considered consistent with the
TMDL provided the NPDES permit requires instream criteria to be met.
TheTMDL, WLA, LA, and MOS are calculated at every 5th flow interval percentile
(Table 5-1).
Table 5-1 Turbidity TMDL based on Total Suspended Solids Calculations for
Sulphur Creek (OK410600010030_00)
Percentile
Flow
(cfs)
TMDL
(lb/day)
WLA (lb/day) LA
(lb/day)
MOS
WWTP MS4 Growth (lb/day)
0 1,668 NA 0 0 NA NA NA
5 45 NA 0 0 NA NA NA
10 19 NA 0 0 NA NA NA
15 12 NA 0 0 NA NA NA
20 9.2 NA 0 0 NA NA NA
25 7.4 1,253 0 0 12.5 1,115 125
30 6 1,016 0 0 10.2 904 102
35 5 847 0 0 8.5 754 85
40 4.2 711 0 0 7.1 633 71
45 3.7 627 0 0 6.3 558 63
50 3.2 542 0 0 5.4 482 54
55 2.7 457 0 0 4.6 407 46
60 2.3 390 0 0 3.9 347 39
65 2.1 356 0 0 3.6 317 36
70 1.8 305 0 0 3.0 271 30
75 1.6 271 0 0 2.7 241 27
80 1.4 237 0 0 2.4 211 24
85 1.2 203 0 0 2.0 181 20
90 1 169 0 0 1.7 151 17
95 0.7 119 0 0 1.2 106 12
100 0 0 0 0 0 0 0
5.7 Reasonable Assurances
ODEQ will collaborate with a host of other state agencies and local governments working
within the boundaries of state and local regulations to target available funding and technical
assistance to support implementation of pollution controls and management measures. Various
water quality management programs and funding sources provide a reasonable assurance that
the pollutant reductions as required by this TMDL can be achieved and water quality can be
restored to maintain designated uses. ODEQ’s Continuing Planning Process (CPP), required by
the CWA §303(e)(3) and 40 CFR 130.5, summarizes Oklahoma’s commitments and programs
aimed at restoring and protecting water quality throughout the state (ODEQ 2006). The CPP
can be viewed from ODEQ’s website at 2006 Continuing Planning Process. Table 5-2 provides
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a partial list of the state partner agencies ODEQ will collaborate with to address point and
nonpoint source reduction goals established by TMDLs.
Table 5-2 Partial List of Oklahoma Water Quality Management Agencies
Agency Web Link
Oklahoma Conservation Commission www.conservation.ok.gov
Oklahoma Department of Wildlife
Conservation
http://www.wildlifedepartment.com/watchabl.htm
Oklahoma Department of Agriculture,
Food, and Forestry
http://www.oda.state.ok.us/aems.htm
Oklahoma Water Resources Board http://www.owrb.ok.gov
Nonpoint source pollution in Oklahoma is managed by the Oklahoma Conservation
Commission (OCC). The OCC works with state partners such as Oklahoma Department of
Agriculture, Food, and Forestry (ODAFF) and federal partners such USEPA and the National
Resources Conservation Service (NRCS), to address water quality problems similar to those
seen in the Study Area. The primary mechanisms used for management of nonpoint source
pollution are incentive-based programs that support the installation of BMPs and public
education and outreach. Other programs include regulations and permits for CAFOs. The
CAFO Act, as administered by the ODAFF, provides CAFO operators the necessary tools and
information to deal with the manure and wastewater animals produce so streams, lakes, ponds,
and groundwater sources are not polluted.
As authorized by Section 402 of the CWA, the ODEQ has delegation of the NPDES
Program in Oklahoma, except for certain jurisdictional areas related to agriculture and the oil
and gas industry retained by State Department of Agriculture and Oklahoma Corporation
Commission, for which the USEPA has retained permitting authority. The NPDES Program in
Oklahoma is implemented via Title 252, Chapter 606 of the Oklahoma Pollution Discharge
Elimination System (OPDES) Act and in accordance with the agreement between ODEQ and
USEPA relating to administration and enforcement of the delegated NPDES Program.
Implementation of point source WLAs is done through permits issued under the OPDES
program.
The reduction rate called for in this TMDL report is 11.7 percent.
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SECTION 6
PUBLIC PARTICIPATION
This report was submitted to EPA for technical review and was technically accepted on
July 01, 2010. A public notice was circulated on July 15, 2010 to local newspapers and/or
other publications in the area affected by this TMDL and persons on the DEQ contact list. The
public comment period ended on August 30, 2010. No requests for a public meeting were
received. No comments were received.
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SECTION 7
REFERENCES
Helsel, D.R. and R.M. Hirsch 2002. Statistical Methods in Water Resources. U.S. Department of the
Interior, U.S. Geological Survey, September 2002.
ODEQ 2006. Continuing Planning Process. 2006 Edition.
ODEQ 2008. Water Quality in Oklahoma, 2008 Integrated Report. 2008
Oklahoma Climate Survey. 2005. Viewed March 6, 2009 in
http://climate.ocs.ou.edu/county_climate/Products/County_Climatologies/
OWRB. 2008. Oklahoma Water Resources Board. 2008 Water Quality Standards.
Tukey, J.W. 1977. Exploratory Data Analysis. Addison-Wesely.
U.S. Census Bureau 2000. http://www.census.gov/main/www/cen2000.html
USEPA 1991. Guidance for Water Quality-Based Decisions: The TMDL Process. Office of Water,
USEPA 440/4-91-001.
USEPA 2003. Guidance for 2004 Assessment, Listing and Reporting Requirements Pursuant to Sections
303(d) and 305(b) of the Clean Water Act, TMDL -01-03 - Diane Regas-- July 21, 2003.
USGS 2007. Multi-Resolution Land Characteristics Consortium. http://www.mrlc.gov/index.asp
USGS 2007a. USGS Daily Streamflow Data. http://waterdata.usgs.gov/nwis/sw
USGS 2009. USGS National Water Information System Website.
http://waterdata.usgs.gov/nwis/?percentile_help
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APPENDIX A
AMBIENT WATER QUALITY DATA
1991 - 2007
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Appendix A
Ambient Water Quality Data
1991 - 2007
WQM Station Date
Turbidity
(NTU)
Total
Suspended
Solids
(mg/L)
Flow
(cfs)
Flow
condition1
410600-01-0030G 9/24/1991 7 15
410600-01-0030G 4/15/1992 5.29 42
410600-01-0030G 10/15/1992 6.29 12
410600-01-0030G 8/4/1993 8 6
410600-01-0030G 6/21/2005 8.01 <10 0.188
410600-01-0030G 7/20/2005 3.56 0.033
410600-01-0030G 7/26/2005 12.7 17
410600-01-0030G 10/11/2005 34.4 27
410600-01-0030G 11/8/2005 31.4 32
410600-01-0030G 12/13/2005 13.5 12
410600-01-0030G 1/24/2006 13.5 <10 Rainfall event
410600-01-0030G 2/28/2006 208 108 0.216
410600-01-0030G 4/4/2006 21.9 15 0.378
410600-01-0030G 5/9/2006 36.3 <10 0.353
410600-01-0030G 6/20/2006 150 <10
410600-01-0030G 10/2/2006 41.5 16
410600-01-0030G 11/6/2006 240 79 20.742 High flow
410600-01-0030G 12/12/2006 19.7 <10 0.511
410600-01-0030G 1/22/2007 55.8 11 21 High flow
410600-01-0030G 2/20/2007 4.64 <10 1.754
410600-01-0030G 3/26/2007 16.8 <10 0.779
410600-01-0030G 5/7/2007 865 1163 High flow
1 High flow = Sample was not collected under base flow conditions (sample collected at flows greater that
25% flow exceedance frequency.
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APPENDIX B
PROJECTED FLOW EXCEEDANCE FREQUENCIES FOR
SULPHUR CREEK FLOW DURATION CURVE
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Appendix B
Projected Flow exceedance frequencies for Sulphur Creek Flow Duration Curve
WBID Segment OK410600010030_00
USGS Gage Reference 07332500
Flow Exceedance
Frequency (%) Flow (cfs)
Flow Exceedance
Frequency (%) Flow (cfs)
Flow Exceedance
Frequency (%) Flow (cfs)
0 1668.2 34 5.2 68 1.9
1 174.5 35 5.0 69 1.9
2 112.9 36 4.8 70 1.8
3 80.3 37 4.7 71 1.8
4 57.9 38 4.5 72 1.7
5 44.7 39 4.4 73 1.7
6 35.4 40 4.2 74 1.6
7 29.2 41 4.1 75 1.6
8 24.8 42 4.0 76 1.5
9 21.7 43 3.9 77 1.5
10 19.2 44 3.8 78 1.5
11 17.3 45 3.7 79 1.4
12 15.7 46 3.6 80 1.4
13 14.5 47 3.5 81 1.3
14 13.3 48 3.4 82 1.3
15 12.5 49 3.3 83 1.2
16 11.7 50 3.2 84 1.2
17 11.0 51 3.1 85 1.2
18 10.3 52 3.0 86 1.1
19 9.7 53 2.9 87 1.1
20 9.2 54 2.8 88 1.1
21 8.8 55 2.7 89 1.0
22 8.4 56 2.7 90 1.0
23 8.0 57 2.6 91 1.0
24 7.7 58 2.5 92 0.9
25 7.4 59 2.4 93 0.9
26 7.1 60 2.3 94 0.8
27 6.8 61 2.3 95 0.7
28 6.6 62 2.2 96 0.7
29 6.3 63 2.2 97 0.5
30 6.0 64 2.1 98 0.4
31 5.8 65 2.1 99 0.12
32 5.6 66 2.0 100 0
33 5.4 67 2.0
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Appendix B
General Method for Estimating Flow at WQM Stations
Flows duration curve will be developed using existing USGS measured flow where the
data exist from a gage on the stream segment of interest, or by estimating flow for stream
segments with no corresponding flow record. Flow data to support flow duration curves and
load duration curves will be derived for each Oklahoma stream segment in the following
priority:
i) In cases where a USGS flow gage occurs on, or within one-half mile upstream or
downstream of the Oklahoma stream segment.
a. If simultaneously collected flow data matching the water quality sample
collection date are available, these flow measurements will be used.
b. If flow measurements at the coincident gage are missing for some dates on
which water quality samples were collected, the gaps in the flow record will be
filled, or the record will be extended, by estimating flow based on measured
streamflows at a nearby gage. First, the most appropriate nearby stream gage is
identified. All flow data are first log-transformed to linearize the data because
flow data are highly skewed. Linear regressions are then developed between 1)
daily streamflow at the gage to be filled/extended, and 2) streamflow at all gages
within 95 miles that have at least 300 daily flow measurements on matching
dates. The station with the best flow relationship, as indicated by the highest r-squared
value, is selected as the index gage. R-squared indicates the fraction of
the variance in flow explained by the regression. The regression is then used to
estimate flow at the gage to be filled/extended from flow at the index station.
Flows will not be estimated based on regressions with r-squared values less than
0.25, even if that is the best regression. In some cases, it will be necessary to
fill/extend flow records from two or more index gages. The flow record will be
filled/extended to the extent possible based on the best index gage (highest r-squared
value), and remaining gaps will be filled from the next best index gage
(second highest r-squared value), and so forth.
c. Flow duration curves will be based on both measured flows only and on the
filled or extended flow time series calculated from other gages using regression.
d. On a stream impounded by dams to form reservoirs of sufficient size to impact
stream flow, only flows measured after the date of the most recent impoundment
will be used to develop the flow duration curve. This also applies to reservoirs
on major tributaries to the stream.
ii) In the case no coincident flow data are available for a stream segment, but flow
gage(s) are present upstream and/or downstream without a major reservoir between,
flows will be estimated for the stream segment from an upstream or downstream
gage using a watershed area ratio method derived by delineating subwatersheds, and
relying on the NRCS runoff curve numbers and antecedent rainfall condition.
Drainage subbasins will first be delineated for all impaired 303(d)-listed WQM
stations, along with all USGS flow stations located in the 8-digit HUCs with
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impaired streams. Parsons will then identify all the USGS gage stations upstream
and downstream of the subwatersheds with 303(d) listed WQM stations.
a. Watershed delineations are performed using ESRI Arc Hydro with a 30 m
resolution National Elevation Dataset (NED) digital elevation model, and
National Hydrography Dataset (NHD) streams. The area of each watershed will
be calculated following watershed delineation.
b. The watershed average curve number is calculated from soil properties and land
cover as described in the U.S. Department of Agriculture (USDA) Publication
TR-55: Urban Hydrology for Small Watersheds. The soil hydrologic group is
extracted from NRCS STATSGO soil data, and land use category from the 2001
National Land Cover Dataset (NLCD). Based on land use and the hydrologic
soil group, SCS curve numbers are estimated at the 30-meter resolution of the
NLCD grid as shown in Table 7. The average curve number is then calculated
from all the grid cells within the delineated watershed.
c. The average rainfall is calculated for each watershed from gridded average
annual precipitation datasets for the period 1971-2000 (Spatial Climate Analysis
Service, Oregon State University, http://www.ocs.oregonstate.edu/prism/,
created 20 Feb 2004).
Table B-1 Runoff Curve Numbers for Various Land Use Categories and Hydrologic Soil
Groups
NLCD Land Use Category
Curve number for hydrologic soil group
A B C D
0 in case of zero 100 100 100 100
11 Open Water 100 100 100 100
12 Perennial Ice/Snow 100 100 100 100
21 Developed, Open Space 39 61 74 80
22 Developed, Low Intensity 57 72 81 86
23 Developed, Medium Intensity 77 85 90 92
24 Developed, High Intensity 89 92 94 95
31 Barren Land (Rock/Sand/Clay) 77 86 91 94
32 Unconsolidated Shore 77 86 91 94
41 Deciduous Forest 37 48 57 63
42 Evergreen Forest 45 58 73 80
43 Mixed Forest 43 65 76 82
51 Dwarf Scrub 40 51 63 70
52 Shrub/Scrub 40 51 63 70
71 Grasslands/Herbaceous 40 51 63 70
72 Sedge/Herbaceous 40 51 63 70
73 Lichens 40 51 63 70
74 Moss 40 51 63 70
81 Pasture/Hay 35 56 70 77
82 Cultivated Crops 64 75 82 85
90-99 Wetlands 100 100 100 100
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d. The method used to project flow from a gaged location to an ungaged location was
adapted by combining aspects of two other flow projection methodologies
developed by Furness (Furness, 1959) and Wurbs (Wurbs, 2000).
Furness Method
The Furness method has been employed in Kansas by both the USGS and Kansas
Department of Health and Environment to estimate flow-duration curves. The method
typically uses maps, graphs, and computations to identify six unique factors of flow
duration for ungaged sites. These factors include:
the mean streamflow and percentage duration of mean streamflow;
the ratio of 1-percent-duration streamflow to mean streamflow ;
the ratio of 0.1-percent-duration streamflow to 1-percent-duration streamflow;
the ratio of 50-percentduration streamflow to mean streamflow;
the percentage duration of appreciable (0.10 ft /s) streamflow; and
average slope of the flow-duration curve.
Furness defined appreciable flow as 0.10 ft/s. This value of streamflow was
important because, for many years, this was the smallest non-zero streamflow value
reported in most Kansas streamflow records. The average slope of the duration curve is
a graphical approximation of the variability index, which is the standard deviation of the
logarithms of the streamflows (Furness, 1959, p. 202-204, figs. 147 and 148). On a
duration curve that fits the log-normal distribution exactly, the variability index is equal
to the ratio of the streamflow at the 15.87-percent-duration point to the streamflow at
the 50-percent-duration point. Because duration curves usually do not exactly fit the
log-normal distribution, the average-slope line is drawn through an arbitrary point, and
the slope is transferred to a position approximately defined by the previously estimated
points.
The method provides a means of both describing shape of the flow duration curve
and scaling the magnitude of the curve to another location, basically generating a new
flow duration curve with a very similar shape but different magnitude at the ungaged
location.
Wurbs Modified NRCS Method
As a part of the Texas water availability modeling (WAM) system developed by
Texas Natural Resources Conservation Commission (TNRCC), now known as the
Texas Commission on Environmental Quality (TCEQ), and partner agencies, various
contractors developed models of all Texas rivers. As a part of developing the model
code to be used, Dr. Ralph Wurbs of Texas A&M University researched methods to
distribute flows from gaged locations to ungaged locations. (Wurbs, 2006) His results
included the development of a modified Natural Resource Conservation Service
(NRCS) curve-number (CN) method for distributing flows from gaged locations to
ungaged locations.
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This modified NRCS method is based on the following relationship between
rainfall depth, P in inches, and runoff depth, Q in inches (NRCS, 1985; McCuen, 2005):
(P I ) S
(P I )
Q
a
2
a (1)
where:
Q = runoff depth (inches)
P = rainfall (inches)
S = potential maximum retention after runoff begins (inches)
Ia = initial abstraction (inches)
If P < 0.2, Q = 0. Initial abstraction has been found to be empirically related to S
by the equation
Ia = 0.2*S (2)
Thus, the runoff curve number equation can be rewritten:
P 0.8S
(P 0.2S)
Q
2
(3)
S is related to the curve number (CN) by:
10
CN
1000
S (4)
P and Q in inches must be multiplied by the watershed area to obtain volumes. The
potential maximum retention, S in inches, represents an upper limit on the amount of
water that can be abstracted by the watershed through surface storage, infiltration, and
other hydrologic abstractions. For convenience, S is expressed in terms of a curve
number CN, which is a dimensionless watershed parameter ranging from 0 to 100. A
CN of 100 represents a limiting condition of a perfectly impervious watershed with zero
retention and thus all the rainfall becoming runoff. A CN of zero conceptually
represents the other extreme with the watershed abstracting all rainfall with no runoff
regardless of the rainfall amount.
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First, S is calculated from the average curve number for the gaged watershed. Next,
the daily historic flows at the gage are converted to depth basis (as used in equations 1
and 3) by dividing by its drainage area, then converted to inches. Equation 3 is then
solved for daily precipitation depth of the gaged site, Pgaged. The daily precipitation
depth for the ungaged site is then calculated as the precipitation depth of the gaged site
multiplied by the ratio of the long-term average precipitation in the watersheds of the
ungaged and gaged sites:
gaged
ungaged
ungaged gaged M
M
P P (5)
where M is the mean annual precipitation of the watershed in inches. The daily
precipitation depth for the ungaged watershed, along with the average curve number of
the ungaged watershed, are then used to calculate the depth equivalent daily flow Q of
the ungaged site. Finally, the volumetric flow rate at the ungaged site is calculated by
multiplying by the area of the watershed of the ungaged site and converted to cubic feet.
In a subsequent study (Wurbs, 2006), Wurbs evaluated the predictive ability of
various flow distribution methods including:
Distribution of flows in proportion to drainage area;
Flow distribution equation with ratios for various watershed parameters;
Modified NRCS curve-number method;
Regression equations relating flows to watershed characteristics;
Use of recorded data at gaging stations to develop precipitation-runoff
relationships; and
Use of watershed (precipitation-runoff) computer models such as SWAT.
As a part of the analysis, the methods were used to predict flows at one gaged
station to another gage station so that fit statistics could be calculated to evaluate the
efficacy of each of the methods. Based upon similar analyses performed for many
gaged sites which reinforced the tests performed as part of the study, Wurbs observed
that temporal variations in flows are dramatic, ranging from zero flows to major floods.
Mean flows are reproduced reasonably well with the all flow distribution methods and
the NRCS CN method reproduces the mean closest. Accuracy in predicting mean flows
is much better than the accuracy of predicting the flow-frequency relationship.
Performance in reproducing flow-frequency relationships is better than for reproducing
flows for individual flows.
Wurbs concluded that the NRCS CN method, the drainage area ratio method, and
drainage area – CN – mean annual precipitation depth (MP) ratio methods all yield
similar levels of accuracy. If the CN and MP are the same for the gaged and ungaged
watersheds, the three alternative methods yield identical results. Drainage area is the
most important watershed parameter. However, the NRCS method adaptation is
preferable in those situations in which differences in CN (land use and soil type) and
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long-term MP are significantly different between the gaged and ungaged watersheds.
The CN and MP are usually similar but not identical.
Generalized Flow Projection Methodology
In the first several versions of the TMDL toolbox, all flows at ungaged sites that
required projection from a gaged site were performed with the Modified NRCS CN
method. This led a number of problems with flow projections in the early versions. As
described previously, the NRCS method, in common with all others, reproduces the
mean or central tendency best but the accuracy of the fit degrades towards the extremes
of the frequency spectrum. Part of the degradation in accuracy is due to the quite non-linear
nature of the NRCS equations. On the low flow end of the frequency spectrum,
Equation 2 above constitutes a low flow limit below which the NRCS equations are not
applicable at all. Given the flashy nature of most streams in locations for which the
toolbox was developed, high and low flows are relatively more common and spurious
results from the limits of the equations abounded.
In an effort to increase the flow prediction efficacy and remedy the failure of the
NRCS CN method at the extremes of the flow spectrum, we developed what is
effectively a hybrid of the NRCS CN method and the Furness method. Noting the facts
that all tested projection methods, and particularly the NRCS CN method, perform best
near the central tendency or mean and that none of the methods predict the entire flow
frequency spectrum well, we decided to adopt an assumption that is implicit in the
Furness method. The Furness method implicitly assumes that the shape of the flow
frequency curve at an upstream site is related to and similar to the shape of the flow
frequency curve at site downstream. As described previously, the Furness method
employs several relationships derived between the mean flows and flows at differing
frequencies to replicate the shape of the flow frequency curve at the projected site,
while utilizing other regressed relationships to scale the magnitude of the curve. Since,
as part of the toolbox calculations, the entire flow frequency curve at a 1% interval is
calculated for every USGS gage utilizing very long periods of record, we decided to use
this vector in association with the mean flow to project the flow frequency curve.
In the ideal situation flows are projected from an ungaged location from a
downstream gaged location. The toolbox also has the capability to project flows from
and upstream gaged location if there is no useable downstream gage.
iii) In the rare case where no coincident flow data are available for a WQM station and no
gages are present upstream or downstream, flows will be estimated for the WQM
station from a gage on an adjacent watershed of similar size and properties, via the same
procedure described above for upstream or downstream gages.
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APPENDIX C
STATE OF OKLAHOMA ANTIDEGRADATION POLICY
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Appendix C
State of Oklahoma Antidegradation Policy
785:45-3-1. Purpose; Antidegradation policy statement
(a) Waters of the state constitute a valuable resource and shall be protected, maintained
and improved for the benefit of all the citizens.
(b) It is the policy of the State of Oklahoma to protect all waters of the state from
degradation of water quality, as provided in OAC 785:45-3-2 and Subchapter 13 of
OAC 785:46.
785:45-3-2. Applications of antidegradation policy
(a) Application to outstanding resource waters (ORW). Certain waters of the state
constitute an outstanding resource or have exceptional recreational and/or ecological
significance. These waters include streams designated "Scenic River" or "ORW" in
Appendix A of this Chapter, and waters of the State located within watersheds of
Scenic Rivers. Additionally, these may include waters located within National and
State parks, forests, wilderness areas, wildlife management areas, and wildlife
refuges, and waters which contain species listed pursuant to the federal Endangered
Species Act as described in 785:45-5-25(c)(2)(A) and 785:46-13-6(c). No degradation
of water quality shall be allowed in these waters.
(b) Application to high quality waters (HQW). It is recognized that certain waters of the
state possess existing water quality which exceeds those levels necessary to support
propagation of fishes, shellfishes, wildlife, and recreation in and on the water. These
high quality waters shall be maintained and protected.
(c) Application to beneficial uses. No water quality degradation which will interfere with
the attainment or maintenance of an existing or designated beneficial use shall be
allowed.
(d) Application to improved waters. As the quality of any waters of the state improve, no
degradation of such improved waters shall be allowed.
785:46-13-1. Applicability and scope
(a) The rules in this Subchapter provide a framework for implementing the
antidegradation policy stated in OAC 785:45-3-2 for all waters of the state. This
policy and framework includes three tiers, or levels, of protection.
(b) The three tiers of protection are as follows:
(1) Tier 1. Attainment or maintenance of an existing or designated beneficial use.
(2) Tier 2. Maintenance or protection of High Quality Waters and Sensitive Public
and Private Water Supply waters.
(3) Tier 3. No degradation of water quality allowed in Outstanding Resource
Waters.
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(c) In addition to the three tiers of protection, this Subchapter provides rules to implement
the protection of waters in areas listed in Appendix B of OAC 785:45. Although
Appendix B areas are not mentioned in OAC 785:45-3-2, the framework for
protection of Appendix B areas is similar to the implementation framework for the
antidegradation policy.
(d) In circumstances where more than one beneficial use limitation exists for a
waterbody, the most protective limitation shall apply. For example, all antidegradation
policy implementation rules applicable to Tier 1 waterbodies shall be applicable also
to Tier 2 and Tier 3 waterbodies or areas, and implementation rules applicable to Tier
2 waterbodies shall be applicable also to Tier 3 waterbodies.
(e) Publicly owned treatment works may use design flow, mass loadings or concentration,
as appropriate, to calculate compliance with the increased loading requirements of this
section if those flows, loadings or concentrations were approved by the Oklahoma
Department of Environmental Quality as a portion of Oklahoma's Water Quality
Management Plan prior to the application of the ORW, HQW or SWS limitation.
785:46-13-2. Definitions
The following words and terms, when used in this Subchapter, shall have the following
meaning, unless the context clearly indicates otherwise:
"Specified pollutants" means
(A) Oxygen demanding substances, measured as Carbonaceous Biochemical Oxygen
Demand (CBOD) and/or Biochemical Oxygen Demand (BOD);
(B) Ammonia Nitrogen and/or Total Organic Nitrogen;
(C) Phosphorus;
(D) Total Suspended Solids (TSS); and
(E) Such other substances as may be determined by the Oklahoma Water Resources
Board or the permitting authority.
785:46-13-3. Tier 1 protection; attainment or maintenance of an existing or designated
beneficial use
(a) General.
(1) Beneficial uses which are existing or designated shall be maintained and
protected.
(2) The process of issuing permits for discharges to waters of the state is one of
several means employed by governmental agencies and affected persons which
are designed to attain or maintain beneficial uses which have been designated
for those waters. For example, Subchapters 3, 5, 7, 9 and 11 of this Chapter are
rules for the permitting process. As such, the latter Subchapters not only
implement numerical and narrative criteria, but also implement Tier 1 of the
antidegradation policy.
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(b) Thermal pollution. Thermal pollution shall be prohibited in all waters of the state.
Temperatures greater than 52 degrees Centigrade shall constitute thermal pollution
and shall be prohibited in all waters of the state.
(c) Prohibition against degradation of improved waters. As the quality of any waters of
the state improves, no degradation of such improved waters shall be allowed.
785:46-13-4. Tier 2 protection; maintenance and protection of High Quality Waters and
Sensitive Water Supplies
(a) General rules for High Quality Waters. New point source discharges of any pollutant
after June 11, 1989, and increased load or concentration of any specified pollutant
from any point source discharge existing as of June 11, 1989, shall be prohibited in
any waterbody or watershed designated in Appendix A of OAC 785:45 with the
limitation "HQW". Any discharge of any pollutant to a waterbody designated "HQW"
which would, if it occurred, lower existing water quality shall be prohibited. Provided
however, new point source discharges or increased load or concentration of any
specified pollutant from a discharge existing as of June 11, 1989, may be approved by
the permitting authority in circumstances where the discharger demonstrates to the
satisfaction of the permitting authority that such new discharge or increased load or
concentration would result in maintaining or improving the level of water quality
which exceeds that necessary to support recreation and propagation of fishes,
shellfishes, and wildlife in the receiving water.
(b) General rules for Sensitive Public and Private Water Supplies. New point source
discharges of any pollutant after June 11, 1989, and increased load of any specified
pollutant from any point source discharge existing as of June 11, 1989, shall be
prohibited in any waterbody or watershed designated in Appendix A of OAC 785:45
with the limitation "SWS". Any discharge of any pollutant to a waterbody designated
"SWS" which would, if it occurred, lower existing water quality shall be prohibited.
Provided however, new point source discharges or increased load of any specified
pollutant from a discharge existing as of June 11, 1989, may be approved by the
permitting authority in circumstances where the discharger demonstrates to the
satisfaction of the permitting authority that such new discharge or increased load will
result in maintaining or improving the water quality in both the direct receiving water,
if designated SWS, and any downstream waterbodies designated SWS.
(c) Stormwater discharges. Regardless of subsections (a) and (b) of this Section, point
source discharges of stormwater to waterbodies and watersheds designated "HQW"
and "SWS" may be approved by the permitting authority.
(d) Nonpoint source discharges or runoff. Best management practices for control of
nonpoint source discharges or runoff should be implemented in watersheds of
waterbodies designated "HQW" or "SWS" in Appendix A of OAC 785:45.
785:46-13-5. Tier 3 protection; prohibition against degradation of water quality in
outstanding resource waters
(a) General. New point source discharges of any pollutant after June 11, 1989, and
increased load of any pollutant from any point source discharge existing as of June 11,
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1989, shall be prohibited in any waterbody or watershed designated in Appendix A of
OAC 785:45 with the limitation "ORW" and/or "Scenic River", and in any waterbody
located within the watershed of any waterbody designated with the limitation "Scenic
River". Any discharge of any pollutant to a waterbody designated "ORW" or "Scenic
River" which would, if it occurred, lower existing water quality shall be prohibited.
(b) Stormwater discharges. Regardless of 785:46-13-5(a), point source discharges of
stormwater from temporary construction activities to waterbodies and watersheds
designated "ORW" and/or "Scenic River" may be permitted by the permitting
authority. Regardless of 785:46-13-5(a), discharges of stormwater to waterbodies and
watersheds designated "ORW" and/or "Scenic River" from point sources existing as
of June 25, 1992, whether or not such stormwater discharges were permitted as point
sources prior to June 25, 1992, may be permitted by the permitting authority;
provided, however, increased load of any pollutant from such stormwater discharge
shall be prohibited.
(c) Nonpoint source discharges or runoff. Best management practices for control of
nonpoint source discharges or runoff should be implemented in watersheds of
waterbodies designated "ORW" in Appendix A of OAC 785:45, provided, however,
that development of conservation plans shall be required in sub-watersheds where
discharges or runoff from nonpoint sources are identified as causing or significantly
contributing to degradation in a waterbody designated "ORW".
(d) LMFO's. No licensed managed feeding operation (LMFO) established after June 10,
1998 which applies for a new or expanding license from the State Department of
Agriculture after March 9, 1998 shall be located...[w]ithin three (3) miles of any
designated scenic river area as specified by the Scenic Rivers Act in 82 O.S. Section
1451 and following, or [w]ithin one (1) mile of a waterbody [2:9-210.3(D)]
designated in Appendix A of OAC 785:45 as "ORW".
785:46-13-6. Protection for Appendix B areas
(a) General. Appendix B of OAC 785:45 identifies areas in Oklahoma with waters of
recreational and/or ecological significance. These areas are divided into Table 1,
which includes national and state parks, national forests, wildlife areas, wildlife
management areas and wildlife refuges; and Table 2, which includes areas which
contain threatened or endangered species listed as such by the federal government
pursuant to the federal Endangered Species Act as amended.
(b) Protection for Table 1 areas. New discharges of pollutants after June 11, 1989, or
increased loading of pollutants from discharges existing as of June 11, 1989, to waters
within the boundaries of areas listed in Table 1 of Appendix B of OAC 785:45 may be
approved by the permitting authority under such conditions as ensure that the
recreational and ecological significance of these waters will be maintained.
(c) Protection for Table 2 areas. Discharges or other activities associated with those
waters within the boundaries listed in Table 2 of Appendix B of OAC 785:45 may be
restricted through agreements between appropriate regulatory agencies and the United
States Fish and Wildlife Service. Discharges or other activities in such areas shall not
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substantially disrupt the threatened or endangered species inhabiting the receiving
water.
(d) Nonpoint source discharges or runoff. Best management practices for control of
nonpoint source discharges or runoff should be implemented in watersheds located
within areas listed in Appendix B of OAC 785:45.